if (document.getElementById) { window.onload = swap };

function swap() {
var numimages=5;
rndimg = new Array("http://www.monashivf.com/Site/DefaultSite/Skins/default/images/header_rotating_1.jpg", "http://www.monashivf.com/Site/DefaultSite/Skins/default/images/header_rotating_2.jpg", "http://www.monashivf.com/Site/DefaultSite/Skins/default/images/header_rotating_3.jpg", "http://www.monashivf.com/Site/DefaultSite/Skins/default/images/header_rotating_4.jpg", "http://www.monashivf.com/Site/DefaultSite/Skins/default/images/header_rotating_5.jpg"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("header").style.backgroundImage = "url("+ randomimage +")"; 
}