Assuming you're familar with scripting, you could use HTML in your global page script if you wanted the background on every page (or just in the page script of the page you want the background image on if you don't want it on every page).
Simply put in this script:
$PAGETEXT := $PAGETEXT + <body background='URL HERE'>
Make sure that there is only one apostrophe on either side of the url you want to put in - for one reason or another (to do with the website coding stuffing up) quotations/two apostrophes won't work.