Hi there,
Is this related to the book somehow? If so, can you please post chapter or page number references? If it isn't related you're better off posting in a more general ASP.NET or CSS category. Makes it easier for you to attract more readers, and easier for readers of this category to find what they came here for.
That said, can you show us the code you're using? This is not something supported by CSS 2 out of the box, so you need to use some CSS tricks. "I feel lucky" for "CSS stretch background image" on Google brings this up, but I am sure there are many other ways to do it (JavaScript comes to mind):
http://webdesign.about.com/od/css3/f/blfaqbgsize.htm
Cheers,
Imar