Is your image a gardient, if not, how exactly do you want to display it so that it's distinguishable AND without causing your site load speed to plummet?
Anyway, here's what you need to do if it's one image that's going to cover your background (again, strongly NOT recommended):
In your CSS file, find the "body" (line 69), and within the curly braces you should see:
Replace it with:
background: HTMLCOLORCODEHERE/transparent url(IMAGE URL HERE);
strings in caps should be replaced with your correspondent info. If you want to apply a background image and repeat it all over the background, us the CSS background-repeat property.