Moving backgrounds on a webpage

I see some websites (in particular lockergnome.com) using a neat trick I'd like to learn.  They have a plain background with a single, large watermark graphic in the background, and when you scroll down the page that watermark slides down along with you.  It moves behind any content on the page and just basically looks very professional.  If anyone can give me the commands or particulars for creating this effect in Dreamweaver MX it would be appreciated.  I have limited experience in HTML and JS, so if I have to create the effect in those I can probably do it with some guidance.  Thanks for your help!