AJAX Thumbnail Rebuild

No. A rebuild writes the resized copies again and leaves the file you uploaded as it is. The only setting that touches an original is "Also optimise the full size original" on the Optimising tab, and even then only the lossless programs run on it - never a re-encode.

It stops where it is. The images it had already got through keep their new copies and nothing is left half written, but the run does not carry on in the background - open the screen and start it again.

Look at the Optimising tab: it lists which optimisers are installed on this server. Those re-pack a file rather than re-encode it, which is where the saving is. Where none of them is installed the plugin falls back to PHP's own image library, and since WordPress already wrote those files at that quality there is usually nothing left to win. On such a host, TinyPNG or ShortPixel will do the job over their API.

Turn on background processing, under Optimising. Optimising a file and writing its WebP and AVIF copies both happen while the browser is still waiting for the upload, and neither has to: with this on the upload finishes as soon as the resized copies are written, and the rest is queued. Action Scheduler runs the queue where a plugin provides it - WooCommerce and many others do - and WP-Cron runs it everywhere else. An image is served in its own format for the minute or two before the queue reaches it.

Yes. The copies are extra files written next to your images; the originals stay exactly where they are, a copy that comes out larger than the original is thrown away, and the copies are deleted along with the attachment. Turning the setting back off puts the front end back to serving the originals. AVIF costs several times the processing WebP does, so expect uploads and rebuilds to take longer with it on, and where the server's image library cannot write AVIF the screen says so and the WebP copies go on being served.

Yes, with one line of code. The screen has a single switch per format, but a filter separates the two, so the copies go on being written while the front end serves your originals - which is what you want when a CDN or the server itself hands the copies out: add_filter( 'ajax_thumbnail_rebuild_serve_copies', '__return_false' );

New uploads are converted as they arrive. For the images already in your library, run a rebuild once.

No - a JPEG can only be replaced by a JPEG, a PNG by a PNG. The attachment keeps its name and its address, which is what keeps every post and product already pointing at it working, and that only holds while the file type stays the same.

Only resized files left on disk that no image size on this site refers to any more - what a removed theme or a size you no longer register leaves behind. It shows you the list and what it would free before anything is deleted, and the files your attachments actually use are never among them.

Yes. The screen talks to the site over a REST API of its own, one image at a time, which is what keeps a large library from running into a script timeout.