But note that many graphics make lousy backgrounds for real pages. Have a look at lousy background for an example. You can also use a simple pictorial image, provided that you use a graphics package to lighten it up. Here's a piece of a Hogarth print , with the colors changed to make it reddish, and then lightened up enough to make the text readable. I'll cover that in class today.
Big files take a long time to download.
A good background graphic should usually be no more than 5K or so.