Quote:
Originally Posted by itimpi
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There will also be a cover.jpg file in the ebook folder, but this will be in addition to one embedded in the ebook. Calibre always keeps an external copy of any cover image as not all formats support embedding the image.
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... and it is vastly faster for calibre to access an separate cover image when needing to display one than it is for it to access any internal cover image a book may have. Thirdly, it is quite possible to have more than one copy of an ebook, in differing formats, in a single record and each can have different internal covers. Calibre needs a way of deciding which to use and having one external image solves that issue.
BTW, while it is preferable to add the cover before converting to ePub, you can add the cover to calibre after the conversion and then do an ePub>ePub conversion to rebuild the ePub integrating that cover. If you still have the FB2 then add the cover calibre and do a new FB2>ePub conversion. If you've lost/deleted the FB2 then add the cover and do an ePub>ePub conversion.