# Regenerate Thumbnails, Correctly Sized > Rebuild all WordPress thumbnail sizes after a theme or settings change. Bulk, single, and background regeneration that won't time out. Free and reliable. - Canonical URL: - Markdown URL: Category: Speed feature Switched themes or changed your image sizes? Rebuild every thumbnail so your images fit their new dimensions - no stretched, blurry, or missing crops anywhere on your site. ## Your Old Thumbnails Don't Match Your New Theme When your image sizes change, WordPress leaves the old files exactly where they were. ### Wrong-Sized Crops A new theme registers new image dimensions, but your existing uploads were only ever cropped for the old ones. Featured images and galleries end up stretched, squished, or awkwardly cut. Impact: Every old image renders at the wrong size ### Missing Image Sizes When a theme or plugin adds a size, WordPress only generates it for images uploaded afterward. Your entire back catalog is left without that size and falls back to a mismatched one. Impact: Hundreds of images with no matching file ### Leftover Files After Cleanup Disable a thumbnail size and new uploads stop generating it - but the old copies stay on disk, quietly eating storage. Impact: 10+ stale files per image left behind ## How Regenerate Thumbnails Works Point it at your library and it rebuilds every size to match your current settings. ### Steps 1. **Scan** — ThumbPress reads each attachment in your media library and checks which thumbnail sizes it should now have. 2. **Rebuild** — It rebuilds every registered thumbnail size from your original file - creating what's missing and replacing what's outdated. 3. **Report** — You get a live tally of total images, processed, created, deleted, not-found, and space saved. ### Two Ways to Get Perfect Thumbnails Pick the exact sizes your site needs, then rebuild your entire library — or a single image — in one click. #### Choose Your Thumbnail Sizes Keep only the image sizes your site needs. During regeneration, disabled thumbnails are deleted, while enabled sizes are generated automatically for every image. #### Regenerate in one click Rebuild every image on your site to match your new settings — run it live with paginated batches, or hand it to a background queue that keeps going after you close the tab. ## Regenerate with Confidence A rebuild that never leaves your library worse than it found it. - Failure-safe - a failed rebuild never wipes metadata - Runs on your own server, nothing sent out - Background queue won't time out on big libraries - Cancel any bulk run mid-way - Live counts for created, deleted, and space saved - Works with any theme, page builder, or WooCommerce ## Frequently Asked Questions ### When do I need to regenerate thumbnails? Any time your image sizes change - after switching themes, editing a size in settings, or disabling sizes you don't use. Regenerating rebuilds your existing images to fit the new dimensions. ### Will this time out on a site with thousands of images? No. Choose background regeneration and ThumbPress processes your library through an Action Scheduler queue that keeps running after you leave the page, so large libraries finish without hitting a PHP timeout. ### Can a failed rebuild break my images? No. Regeneration is failure-safe: if a single image can't be rebuilt, its existing metadata is left untouched rather than wiped, so you never end up with a blank or broken thumbnail. ### Does regenerating delete my old thumbnail sizes? It rebuilds every size your site currently registers. Sizes you've disabled are cleaned up, and the run reports exactly how many files were created and deleted along with the space saved. ### Can I regenerate just one image? Yes. There is a regenerate button on the individual attachment, so fixing a single wrong crop does not mean reprocessing your entire library. ### Will regenerating change my image URLs? No. The original file and its URL are untouched - only the generated sizes are rebuilt - so nothing you have already published breaks. ### Can I cancel a background run? Yes. Cancelling stops the queue cleanly, and images already processed keep their new thumbnails. You can start a fresh run whenever it suits you. ### Does it need to re-upload or re-download anything? No. Everything is rebuilt on your own server from the original file that is already there, so there is no transfer and no external service involved. ### How long does regenerating a large library take? It depends on your server and image sizes, but background regeneration works through the library in batches with live progress, so you can leave it running and check back rather than watching a browser tab. ## Ready to Rebuild Your Thumbnails? Install ThumbPress free and rebuild every thumbnail size in a few clicks. ## About ThumbPress ThumbPress is a WordPress plugin for image optimization and thumbnail management, built by Codexpert. It runs on your own server: compression, WebP/AVIF conversion and cleanup all happen locally, with no third-party image API and no per-image credits. - All features: - Pricing: - Documentation: - Blog: - Support: - WordPress.org listing: