[icearch]

How does cover work in epub?

I mean, how do software determine what to display as thumb nail?

I am being lazy and use a old file to save as a new file to make a new book, since they share same structure. Then I just sawp the cover image in Images folder to a new one, leaving cover.xhtml as it be.

Previewing in sigil looks fine, and getting into the book shows cover correctly as well. But thumb nail in reading app becomes random image in the file, or just do not display at all.

How does this happen? Where do I left unchageed?