If you create Kindle books with KindleGen or Calibre, you'll need to mark the cover with a special metadata entry:
Code:
<meta content="cover" name="cover" />
with a corresponding entry in the manifest section:
Code:
<item href="Images/cover.jpeg" id="cover" media-type="image/jpeg" />
You may want to look into
Sigil, which will allow you to automatically add the required metadata and guide items via the Add Semantics menu.