WordPress Core
When you upload an image to WordPress, the system automatically generates multiple resized versions called thumbnails. These intermediate sizes serve different layout contexts — the original full-size file is stored alongside smaller variants for use in listing pages, featured image slots and responsive image sets.
How WordPress generates thumbnails
WordPress ships with three default sizes: thumbnail (150×150 cropped), medium (up to 300×300) and large (up to 1024×1024). Your active theme and installed plugins can register additional sizes using add_image_size(). All generated variants are stored in the same /wp-content/uploads/ directory as the original.
Each thumbnail variant is stored as a separate file but is not a separate attachment in the database — metadata about the variants is stored in wp_postmeta under the key _wp_attachment_metadata, as a serialised array of generated sizes.
Thumbnails and storage
On a site with a large library, thumbnail generation can multiply storage use significantly. If your theme registers ten custom sizes, each uploaded image generates ten additional files. Deregistering unused sizes using remove_image_size() and running a thumbnail regeneration pass removes the unneeded variants.
Mediapapa does not currently manage thumbnail variants directly — its compression and governance applies to the source attachment. Thumbnail regeneration is handled by WordPress core or dedicated plugins like Regenerate Thumbnails.
Frequently asked questions
Are image thumbnails separate attachments in the Media Library?
No. Thumbnail variants are stored as files on disk but are not separate attachment records in the database. The Media Library only shows the original upload. The variants are tracked in the attachment’s postmeta.
What happens to thumbnails if I delete an image?
When you delete an attachment from the Media Library, WordPress deletes the original file and all its thumbnail variants from disk. The database record is also removed.
Do thumbnail sizes affect page performance?
Yes. Using an oversized image in a context that only needs a small thumbnail loads unnecessary data. WordPress’s responsive image support (srcset) helps browsers choose the right size, but only if the appropriate thumbnail variants exist.
Related terms: Responsive Images · srcset · Image Optimisation
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