Covers are defined by a meta element in the content.opf file within the metadata section. The meta element needs to have two attributes, “name” which needs to be cover and “content” which points to the id of the cover item (needs to be an image) in the manifest section.
<meta content="
cover.jpg" name="cover" />
http://john.nachtimwald.com/2011/08/...ot-showing-up/