We use one for this site, and if you do a search, I bet you'll get lots of hits on this topic.
I'd strongly advise against it though. It is bad for usability and accessability. If not done correctly, it will hurt your search engine rating.
/Steve
I agree with Steve, don't use images for navigation in any site.
This is the direction we all should go towards instead.
http://www.robertnyman.com/2006/09/20/the-web-standards-war-is-far-from-over/
/HAXEN