I believe it's called
doc_background in your Stylevars. You would add
fixed to the CSS code.
Code:
colortype url(location/of/image/on/server.jpg) center center fixed
In you additional.css template, you can add
background-size:cover to fill the whole page with your image, but this isn't supported in older versions of IE. You might want to settle on an image that's large enough to fill the screen when people browser with these super giant monitors they have these days, but optimized enough so as not to chew up resources.