Why Thumbnails Stay JPG After WebP Optimization (And How to Fix It)

The Problem: Optimization Ran, But Thumbnails Are Still JPG

What the Client Sees

After running LiteSpeed’s image optimization service and confirming that full-size images are being served as WebP, a common discovery is that thumbnail images — the smaller cropped versions that appear on archive pages, blog indexes, and category pages — are still being served as JPG. The client sees this as incomplete optimization. The developer needs to explain why it happens and how to fix it.

On a travel website with hundreds of walking route posts, the archive page is often one of the highest-traffic pages on the site. If the archive page thumbnails are still loading as JPG while the individual post pages serve WebP, the performance improvement is inconsistent — and it shows up in PageSpeed Insights as an “images not in next-gen format” recommendation.

Why WordPress Generates Separate Thumbnail Files

When an image is uploaded to WordPress, the platform automatically generates multiple resized versions of that image based on the registered image sizes in the active theme. A standard WordPress installation generates at least three sizes by default: thumbnail (150×150), medium (300×300), and large (1024×1024). Themes and plugins often register additional sizes.

These resized versions are separate physical files stored in the same upload directory as the original. A single uploaded image named photo.jpg might result in photo-150x150.jpg, photo-300x200.jpg, photo-700x458.jpg, and photo-1024x682.jpg — all stored as separate JPG files. LiteSpeed’s optimization service converts the full-size image and its standard variations, but WordPress-generated crop sizes with specific pixel dimensions are sometimes missed, depending on how the optimization was configured and when the images were originally uploaded.

Diagnosing Which Thumbnails Are Not Converted

Using Browser DevTools to Identify JPG Thumbnails

Open the archive or category page in a browser, open developer tools, and switch to the Network tab. Filter by image type and reload the page. Any image with a Content-Type of image/jpeg is being served as JPG. Note the filenames — they will typically include dimension suffixes like -700x458 or -150x150.

Compare these filenames against what exists in the server’s upload directory. If a WebP version of the thumbnail exists at photo-700x458.jpg.webp, the optimization ran but the file is not being served correctly. If no WebP version exists for that size, the thumbnail was never included in the optimization queue.

The Fix: Manual Cleanup via FTP

When to Use FTP for Image Cleanup

When specific thumbnail sizes are not converting correctly and the issue cannot be resolved through LiteSpeed’s dashboard, the most reliable solution is to delete the problematic JPG thumbnail files directly from the server via FTP. WordPress will not regenerate these files automatically after deletion — they are only regenerated when the original image is re-uploaded or when a regenerate thumbnails plugin is run. Once the original JPG thumbnail is deleted, browsers requesting that size will be served the full-size WebP instead, which is typically still an improvement over the original JPG thumbnail.

When accessing the server via FTP, be precise about which files to delete. The rule is: delete only the dimension-suffixed JPG files — those with filenames like photo-700x458.jpg — and leave all other files intact. Do not delete the base original file, the LiteSpeed optimization backup (which typically has a .bk extension or is stored in a separate directory), or any WebP versions that already exist.

Files to Delete vs Files to Leave

Delete: dimension-suffixed JPG files such as -700x458.jpg, -150x150.jpg, -300x200.jpg.

Leave: the base image file (photo.jpg), any WebP versions (photo.jpg.webp, photo-700x458.jpg.webp), and any LiteSpeed backup files.

Verifying the Fix

After deleting the JPG thumbnails, clear the LiteSpeed Cache and reload the archive page. Check the Network tab in developer tools again. The thumbnail images should now show a Content-Type of image/webp. If any JPG thumbnails remain, repeat the identification process and confirm that the corresponding files were deleted from the server.

The next article covers what happens after the optimization is complete and the client wants to reclaim server storage by deleting the original image backups — including what to check before proceeding and what should never be deleted.