In the BODY class of the styles.css put this:

Code:
background: url(../images/yourbackgroundimage.jpg) repeat-y 50% 0 #ffffff;

background-attachment:fixed;
Note the background image properties for the repeat and color...change that to suit your site (whether it's no-repeat, repeat-x, repeat-y or whatever color the background would be if the image wasn't there).

Also: you *may* have to set the width as well as the max and min width of the page as a whole. I like to use 1000px for the width of pages.