If WordPress saves the original upload but does not create thumbnail, medium, large, or theme-specific files, the failure is usually in image processing rather than the upload itself. WordPress needs a usable GD or Imagick library, enough PHP and server resources, writable upload directories, and a registered image size. A theme or plugin can also request a size that was never registered.
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Work through the checks in order: confirm which files are missing, inspect Tools > Site Health > Info, correct the server or settings problem, test with a small ordinary image, and then regenerate existing attachments. Regeneration cannot repair a server that still cannot process or write images.
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When you upload an image, WordPress normally keeps the original in the uploads directory and creates additional files for its built-in sizes and any sizes registered by the active theme or plugins. A 150-by-150 crop may appear with a filename such as photo-150x150.jpg; medium, large, and custom sizes use similar suffixes.
First establish the exact symptom in the Media Library and on the server:
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- The original attachment exists, but expected
-150x150, medium, large, or theme-specific files do not. - Only some sizes are absent. This can mean a particular size was never registered, rather than that all processing failed.
- Every generated size is absent. That points more strongly to a missing image library, insufficient resources, permissions, an unsupported format, or code that prevents generation.
- A theme displays a broken image while other sizes exist. The theme may be requesting a size that was never registered or regenerated.
Do not assume that a missing file proves a failed upload. A size can be absent because it was added after the image was uploaded, because the active theme changed, or because a plugin filters the generation process.
How WordPress creates image sizes
Thumbnail creation is server-side image processing unless the browser-side processing path described below is available. WordPress hands the source image to GD or the Imagick PHP extension, allocates memory to decode and resize it, writes the result into the uploads directory, and records the generated metadata with the attachment.
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Both the source dimensions and the format affect the work required. GD image functions are memory intensive, and large, animated, or unusual files can exhaust memory or exceed execution time even when a small JPEG works. Imagick depends on an ImageMagick installation behind the PHP extension. If neither usable GD nor Imagick is available, WordPress cannot create normal derivative sizes.
WordPress may request additional memory for image processing, but that request cannot overcome every host limit. A constrained shared server, a low PHP memory_limit, a short execution time, or restrictive process limits can still stop the operation.
Follow this diagnostic sequence
1. Confirm the missing files and the requested size
- Open the attachment in Media > Library and confirm that the original image opens.
- Inspect the corresponding uploads directory if you have file access. Look for the expected suffixes, such as
-150x150, and for the medium, large, or custom names used by the site. - Identify which component requests the missing size. Check the active theme and relevant plugins for a registered name, rather than treating every absent derivative as a processing failure.
If the original is present but one custom size has never been registered, uploading the image again will not create that size. Registration or regeneration is the appropriate remedy.
2. Inspect Site Health before changing code
Go to Tools > Site Health > Info. Review the Media Handling and Filesystem Permissions sections first. WordPress documents the GD version and supported formats, whether Imagick is available, its version, PHP variables, upload limits, and whether the directories it needs are writable.
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Record the information rather than relying on a green or red summary alone. The exact library, version, supported formats, PHP limits, and writable-path status determine which branch of the fix applies.
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3. Check PHP and server capacity
Ask the host or inspect the server configuration for these variables and components:
memory_limit, which bounds PHP memory available while an image is decoded and resized.upload_max_filesizeandpost_max_size, which can reject a source before processing starts.max_execution_timeand any web-server timeout, which can terminate a slow resize.- GD with the functions WordPress needs, or Imagick with a working ImageMagick installation.
- Available CPU and memory on the hosting account, especially during large-image uploads or bulk regeneration.
There is no universal “correct” memory number. Required capacity depends on pixel dimensions, color depth, compression, format, and the server configuration. A high-megapixel image can require far more working memory than its compressed file size suggests. Increasing a PHP value only helps when the host permits it and the underlying process still has enough resources.
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WordPress must be able to create files in the year-and-month uploads directory and update attachment metadata. A directory can exist yet be unwritable because of ownership, permissions, a read-only filesystem, a security policy, or a full disk/quota.
Site Health is the fastest first check. If it reports a permissions problem, have the administrator or host correct ownership and write access for the web process. Do not “fix” this by making the entire site world-writable; use the host’s supported ownership and permission model. After the correction, test a new upload and confirm that derivative files appear.
5. Check Media settings and registered sizes
Open Settings > Media and review the built-in Thumbnail size, Medium size, and Large size values, including whether thumbnails are cropped to exact dimensions. These settings control what WordPress generates going forward.
Then inspect the active theme and plugins for add_image_size() declarations. Confirm that the theme calls add_theme_support( 'post-thumbnails' ) for the relevant post type and that a custom post type is configured to support thumbnails when applicable. A size added after an image was uploaded does not appear retroactively. Existing attachments need regeneration after the registration is correct.
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6. Test with a small, ordinary image
After correcting a reported server issue, upload a small RGB JPEG or PNG with ordinary dimensions. This isolates the variables:
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- If the small file succeeds but a very large, animated, or unusual file fails, resource use or format support is the likely distinction.
- If every file fails, concentrate on GD/Imagick availability, write permissions, PHP limits, and code that filters image-size generation.
- If only one theme size fails, verify that the name and crop declaration are valid and that the active theme is the one you inspected.
Review PHP and web-server error logs for the time of the test when they are available. Look for memory exhaustion, timeout, unsupported-format, or permission errors. Do not label a plugin conflict without a reproducible failure or a log entry that supports it.
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GD or Imagick is missing or unusable
Contact the host to install or repair the PHP GD extension, or to enable Imagick with a working ImageMagick installation. Ask which PHP version the site runs and whether the extension is loaded for that version; an extension installed for a different PHP handler will not help WordPress.
Once the host confirms the library, use Site Health to verify the detected version and supported formats, then run the small-image test. Re-uploading before the extension is fixed only creates more originals without derivatives.
The uploads directory cannot be written
Have the host correct the directory owner, group, and permissions for the web process, and check disk space and account quota. Verify the exact path shown by WordPress rather than changing an unrelated directory. Test a new image after the fix; successful creation of both the original and its derivatives confirms that the write path is working.
PHP memory or execution limits are too low
Provide the host with the failing image dimensions and the log message, if any. The host may raise memory_limit, max_execution_time, or related process limits, move the site to a plan with more CPU and memory, or impose a practical upload-dimension policy.
Do not promise that one numeric setting works for every site. A large source can still fail at a value that handles ordinary images, and raising a limit does not repair a missing library or unwritable directory. Keep the original dimensions, format, and observed error together when discussing the change.
The format is unsupported or unusually expensive
Test a standard RGB JPEG or PNG. If that works, the failing format may not be supported by the installed library, or it may require more memory than the host provides. Animated and unusually large images are common stress cases. Convert the source before upload when the site does not need the original format, or ask the host whether its GD/Imagick build supports that format.
A size is not registered, or registration changed
Correct the add_image_size() declaration, thumbnail support, or custom post-type support in the active theme or plugin. Check spelling and the requested size name in the template. Then regenerate existing attachments; registration affects future processing but does not manufacture files for images already in the library.
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Theme or plugin code filters generation
Use the small-image test and logs to establish whether all sizes or one named size is affected. Review code that filters intermediate image sizes or changes the upload path. On a staging copy, temporarily use a default theme and disable nonessential image-related plugins, one change at a time. If generation returns, re-enable components individually to identify the responsible code, then update or reconfigure it rather than leaving unrelated plugins disabled on production.
WordPress 7.1 and browser-side processing
WordPress 7.1 introduces client-side media processing with WebAssembly and libvips in supported browsers. When that path is available, resizing and format conversion can move away from PHP, reducing server CPU and memory pressure for the upload operation.
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This is not a universal replacement for server repair. An unsupported browser, insufficient device resources, or disabled client-side processing falls back to traditional server-side GD or Imagick. A site whose fallback path lacks a library, cannot write uploads, or runs out of memory still needs a host-side fix. Test in the browsers your editors actually use and verify the generated files on the server.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Regenerate thumbnails after the cause is fixed
Regeneration operates on existing attachments; it does not correct a broken image-processing environment. Back up the database and uploads before a bulk operation, run a small subset first on production, and schedule the work when server load is acceptable.
Regenerate the complete library with WP-CLI
Administrators with shell access can run:
wp media regenerate --yesWP-CLI reports each attachment as it processes it. Monitor the command output and server resource usage. If it fails, stop and fix the reported library, permission, format, or resource problem before launching another bulk run.
Regenerate one registered size
When a theme added or changed only one size, limit the operation:
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wp media regenerate --image_size=thumbnail --yesReplace thumbnail with the registered image-size name you need. The selector is useful when a full-library run would create unnecessary load, but it still requires the size to be correctly registered and the server to be able to process the source images.
Use a dashboard tool when shell access is unavailable
A vetted thumbnail-regeneration plugin can provide a dashboard workflow. Choose one that clearly states which attachments and sizes it will process, supports a dry run or limited batch where possible, and has been maintained for your WordPress version. Back up first and regenerate a small selection before sending the entire library through it.
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Choose the remedy by scope and risk
| Situation | Best first action | Scope | Main operational concern |
|---|---|---|---|
| One attachment is missing a derivative | Check its source format, dimensions, and attachment metadata; then regenerate that attachment with an approved tool. | Single attachment | Do not treat an isolated format or resource failure as a site-wide outage. |
| One registered size was added or changed | Verify the registration and run WP-CLI with --image_size or an equivalent limited dashboard job. |
Selected size | The size must exist in the active theme or plugin before regeneration. |
| Every new upload lacks derivatives | Inspect Site Health, libraries, permissions, PHP limits, and logs before any bulk job. | Future uploads first | Bulk regeneration will repeat the same failure until the root cause is fixed. |
| A theme migration changed many sizes | Back up, test a small batch, then regenerate the affected library during a low-traffic period. | Whole library or selected sizes | CPU, memory, disk space, and production timing can affect visitors and editors. |
Verify that the repair worked
- Upload a small RGB JPEG or PNG and confirm the original plus expected built-in and theme-specific files are present.
- Open a representative page for each post type that uses thumbnails and confirm the requested size resolves without a broken-image response.
- Check the attachment metadata and uploads directory for the generated dimensions and filenames.
- Test a larger image separately, because success with a small file does not prove that the server can process every source.
- Review logs after the test and after regeneration so a hidden timeout or memory error is not mistaken for success.
Keep a record of the active theme’s registered sizes, the server’s image libraries, and any host limits. That makes a future theme change or migration easier to diagnose.
What the 2560-pixel threshold does—and does not do
WordPress documents a default big_image_size_threshold of 2560 pixels. This behavior can cause WordPress to scale an especially large upload before it creates other derivatives. It is a processing rule, not a promise that every image above or below that dimension will succeed.
Actual success still depends on the source format, pixel dimensions, available memory and CPU, execution time, image library, and writable storage. Treat the threshold as one setting to understand when inspecting a large upload, not as a universal server requirement or a guaranteed workaround.
Prevent recurring thumbnail failures
- Keep GD or Imagick enabled and verify it after PHP or hosting changes.
- Monitor disk space, account quota, PHP memory, and execution time on sites that receive large uploads.
- Document custom sizes and post types so a theme replacement does not silently request unregistered derivatives.
- Use representative small and large test images after changing the theme, PHP version, image library, or hosting plan.
- Back up before bulk regeneration and limit the batch when production resources are tight.
- Ask editors to avoid unnecessarily huge or unusual source files when the site does not need their full dimensions.
Frequently Asked Questions
Why does the original image exist while the 150-by-150 thumbnail is missing?
The original can be written successfully even when derivative processing fails. Check the image library, resources, permissions, and whether the 150-by-150 size is registered; then regenerate after the cause is corrected.
Will changing Settings > Media recreate thumbnails for old uploads?
No. Media settings control processing going forward. Existing attachments need a regeneration run after the new dimensions or crop behavior are saved.
Is Imagick always better than GD for WordPress thumbnails?
Neither is universally better. Both can process images when correctly installed; the result depends on supported formats, versions, memory, CPU, and host configuration.
Can WordPress 7.1 browser processing fix a broken server?
Only for uploads that successfully use the supported-browser client-side path. Unsupported browsers and failed client processing fall back to GD or Imagick, so the server path must still work.
What should I do if a small JPEG works but a large image fails?
Treat it as a capacity or format issue. Compare dimensions and format, inspect memory and timeout limits and logs, and ask the host to adjust resources or establish a practical upload policy.
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