3
2 ways to go about it...
customize the templates for each module, inserting each into a table with a different background image.
or
get
RID, throw it into /themes/yourtheme/backgrounds/ along with the images you would like it to alternate thru, & call it from your css like
body {background-image: url('backgrounds/rid.php?pic=random1';}
note i havent tested either of these options, but i think theyd work for what u have in mind.
hope this helps.