Image backround on main page.

[Logikos]

Ok i know you might think this is a question.
"How do i put an image on my backround for my main page"
i know how to do that
heres the tricky part..

I want an image that i made placed on my main forums backround, but i dont want it to move when the users scroll down to see the diffent forums. I want that back round image to stay put. I know this is possiable cause i've seen it done here at http://www.zelaron.com/forums/index.php?styleid=73 just change to the "X79" style on the main pages drop down box and you will see what im talking about. The image that he has doesnt move when you scroll down. Can someone please tell me how to do this. And where to place it.

thanks

-Nem