if (document.getElementById) { window.onload = swap };
function swap() {
var numimages=16;
rndimg = new Array("images/header_viu.jpg", "images/header_fn.jpg", "images/header_pda.jpg", "images/header_cad.jpg", "images/header_dm1.jpg", "images/header_dm2.jpg", "images/header_fish.jpg", "images/header_globe.jpg", "images/header_leaves.jpg", "images/header_log.jpg", "images/header_rock1.jpg", "images/header_rock2.jpg", "images/header_rock3.jpg", "images/header_sat.jpg", "images/header_tron.jpg", "images/header_vi.jpg"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("header").style.backgroundImage = "url("+ randomimage +")"; 
}