Mobi2mobi (GUI) version allows you to edit the meta data in ebooks and add covers even if the book is dedrm'd. You need to download the book to your computer and change the extension on the book to .mobi (I know your computer gives you a warning about the book becoming unstable-but azw is mobi)
Then go to librarything.com and look up a book cover you like. Save the image on your computer. Open mobi2mobi (GUI) add the cover and save. Should work.