if (document.getElementById) { window.onload = swap };
function swap() {
var numimages=4;
rndimg = new Array("images/site_footer1.gif", "images/site_footer2.gif", "images/site_footer3.gif", "images/site_footer4.gif"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("footer").style.backgroundImage = "url("+ randomimage +")"; 
}