if (document.getElementById) { window.onload = swap };
function swap() {
var numimages=2;
rndimg = new Array("imagenes/banner-01.gif", "imagenes/banner-02.gif"); 
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("banner").style.backgroundImage = "url("+ randomimage +")"; 
}