if (document.getElementById) { window.onload = swap };
function swap() {
var numimages=12;
rndimg = new Array("images/banner1.jpg", "images/banner2.jpg", "images/banner3.jpg", "images/banner4.jpg", "images/banner5.jpg", "images/banner6.jpg", "images/banner7.jpg", "images/banner8.jpg", "images/banner9.jpg", "images/banner10.jpg", "images/banner11.jpg", "images/banner12.jpg"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("banner").style.backgroundImage = "url("+ randomimage +")"; 
}