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

function swap() {
var numimages=3;
rndimg = new Array("images/testimonials/02.gif", "images/testimonials/03.gif", "images/testimonials/04.gif"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("header").style.backgroundImage = "url("+ randomimage +")"; 
}