Mobi books have a cover feature, kind of like a form of meta-data like the title of the book or author. PDFs on the other hand don’t really have a dedicate “cover image” feature. One makes a cover page by creating a page that is an image from edge to edge—that’s all it is really, an unusual page. If you have an image that is the right size and use that technique, it should come out just like you’d expect it to look. I got a pretty good result out of it.