To make your background not scroll with your forum index do the following:
Styles and Templates > Style Manager
Choose your theme/style and click on Go
In the input box at the top it will say "search stylevar", in there look for "doc_background"
Once you are in there add the following:
At the very end of your background's url add "fixed" , minus the double quotes.
It should look like this:
Code:
url(http://www.yourdomain.com/path-to-your-background-image/image.jpg) fixed
the image type could also be image.png, image.gif, what ever it is that your background image is.
You shouldn't have to change anything except just adding "fixed" to the end of the url.
Then make sure you have
Background Repeat set to "repeat-y"
Units set to "% "
Background Image Horizontal Offset set to "50"
Background Image Vertical Offset set to "0"