Calehorn - Thanks for the advice. There are three difficulties in deleting the thumbnails and then the epubs: The books that are only grey boxes change - there will be the regular cover when I add the book and then it will be a box the next day; I would have to copy all of those thumbnail files to another folder, then change the _png to .png to see what the book is; and finally, where are the other 600 books' thumbnails.
At least most of the books now have correct covers that when I first posted the problem.
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