For ePub2 or ePub2 fallback, this is the code in the OPF you use to specify the cover. The top line goes in the metadata.
Code:
<meta name="cover" content="coverimg"/>
For ePub3, you use the following. For ePub2, you remove the properties.
Code:
<item href="images/9780399593550_cover.jpg" id="coverimg" media-type="image/jpeg" properties="cover-image"/>