[Digital Jedi]

Presuming you've applied the background image to an element with CSS, you just set the background-attachment to fixed:

background-image:url('mybackground.gif');
background-repeat:no-repeat;
background-attachment:fixed;

Then you can set the position of the element, if you don't want it to default to the top right corner.

Or you can use it in the shorthand version, which is, essentially, in this order:background-color background-image background-attachment position-x position-y:

background:#FFFFFF url('img_tree.png') no-repeat center top