Some related points here:
https://audiobookbay.lu/forum/guides/how-to-reseed/For a missing image file, often it's the same image used as the torrent cover image, so see if that matches (rename it to match the image in the torrent, then do a force recheck to see if it is the same as the original).
And if you have an MP3 file missing a bit at end or beginning, you may be able to salvage it by loading it in MP3DirectCut, checking the beginning and ending then saving. If it's stuff about the publisher, you can cleanly cut that and still have the complete book. Or there may be metadata, such as a cover image, which you can add using MP3tag.
But never, ever re-up an incomplete file.