if (document.getElementById) { window.onload = swap };
function swap() {
var numimages=4;
rndimg = new Array(
	"/css/forestry/images/header_01.jpg",
	"/css/forestry/images/header_02.jpg",
	"/css/forestry/images/header_03.jpg",
	"/css/forestry/images/header_01.jpg"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("header").style.backgroundImage = "url("+ randomimage +")"; 
}