If your book cover looks stretched around the book, you probably loaded just a front image, not a full cover one.
Uncheck the "Use solid cover image" checkbox off to switch to separated cover mode.
See here for more details: http://boxshot.com/support/boxshot-help/shapes_book.html
Please note that CD/DVD boxes and Binders don't support this feature. Feel free to submit requests for them, as well. Here is a request for DVD boxes, for example.