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Paula59 - as I understand it, what you want to do is to create book files of different formats for various distributors with different cover image files. Assuming that's true, I would try something like this:
- Put the different images inside an archive with names like apple.jpg, kobo.jpg, mobi.jpg, generic_epub.jpg etc.
- add the archive to the book, drag and drop into the Book Details sidebar will do the trick
Calibre will auto-rename a zip archive full of images from .zip to .cbz, same for a solid image .rar archive, it'll be renamed to .cbr. However, you can workaround the auto-renaming by including a non-image file in a zip or rar archive. Or you could put the images into something like a .7z archive, it won't get renamed.
- Before creating a book file for a particular distributor in a particular format open the archive (either using View specific format, or by clicking it in the Book Details sidebar), drag the required image out of the archive to the desktop, and then drag that image from there to the Book Details sidebar. This has to be a two step process because calibre will not accept an image when it's dragged and dropped into book details from an archive.
- Create the book file via Save to Disk,
BR