if (document.getElementById) {window.onload = swap};
function swap() {
var numimages=4;
rndimg = new Array("01.jpg", "02.jpg", "03.jpg", "04.jpg"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("maincontent").style.backgroundImage = "url(images/background/"+ randomimage +")"; 
}