Quote:
Originally Posted by DSpider
Epub is basically a zip archive. If it's DRM-free, open it with WinRAR, 7-Zip, etc., and replace de JPG image (with the same filename, resolution, etc).
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Doing this you can create an EPUB that cannot be opened. The EPUB spec requires the elements to be in a specific order inside of the ZIP archive. If the application you're using changes the order by decompressing to a temporary location and recompressing you will end up with a book that cannot be opened.
JSWolf is correct that EPUB to EPUB conversation is unnecessary because you can edit the XML and add a cover and proper references to yourself. However, in cases where an EPUB does not have a cover properly defined it's easier for most people to do a one click convert than it is to fiddle with XML.