Just for future ref, if anyone's wondering, you use the URL and IMG tags to nest your cover image inside the link, e.g.
[URL="http://yourbooklink.htm"][IMG]http://yourimagelink.jpg[/IMG][/URL]
so if anyone clicks on the image, they're taken to the linked address.
This is probably already somewhere in FAQs forum, but one more never hurts!
-Derek