[setishock]

In the BODY css attribute put a filler color to cover any area that the image does not cover.
In the BODY Extra CSS Attributes you'll need to add something like this:

background-image: url(http://myforum.com/directory/path/to/the/image);
background-repeat:no-repeat;
background-attachment:fixed;
background-position:center;

No-repeat will cause the image to only show one time. Fixed nails the image down so the forum glides over it when scrolling. And center puts it in the center of the left and right borders.