if (document.getElementById) { window.onload = swap };
function swap() {
var numimages=6;
rndimg = new Array("/images/01.jpg", "/images/02.jpg", "/images/03.jpg", "/images/04.jpg", "/images/05.jpg", "/images/06.jpg"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("banner").style.backgroundImage = "url("+ randomimage +")"; 
}