[setishock]

You can go about it in two ways.
1a > acp > styles and templates > style manager > select - all styles options (for the theme you want to work on)
1b > Scroll down to Body > where the color is replace that with a url to an image like this:

 url(http://www.yourforum.com/path/to/image) repeat-x no repeat

Make sure there's an image where the url points to. When I'm putting a theme together I park an image in the albums and use that url. Makes testing easy.
The alternate is:
2a > Leave the color data as is, to act like a filler. Just in case the image doesn't fit right in some browsers.
2b > Add to the extra css attributes on the right something like this:

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

I hope this is what you had in mind. To see this in action go to http://animeappeal.com/forums/index.php?styleid=36