How do you add an image to a template as a background?

[setishock]

And for those reading this thread that also would like to know how he fixed the image scrolling with the forum, here's the way to do it.
At the end of the image url add the word > fixed. Like this:

url(path/to/your/image) fixed

That pins the image down so when you scroll down the forums the forum slides over the background image. In some cases where the image is not big enough, you may also want to set the repeat to no repeat and use a filler color found at the top of the stylevar.