find the id of the forum then add some css to your additional.css template.
Code:
#forum4 {
background-image: url(left.png), url(right.png);
background-repeat: no-repeat;
background-position: left top, right top;
}
google CSS Multiple background images for more information. The 4 above is the example forum ID
--------------- Added [DATE]1317484486[/DATE] at [TIME]1317484486[/TIME] ---------------
http://www.css3.info/preview/multiple-backgrounds/