var id="header";

//qui sotto tra virgolette andranno indicati i
//nomi delle immagini separate da una virgola.

var imgArray=new Array("img/banner_top_img1.jpg", "img/banner_top_img2.jpg", "img/banner_top_img3.jpg", "img/banner_top_img4.jpg", "img/banner_top_img5.jpg", "img/banner_top_img6.jpg", "img/banner_top_img7.jpg", "img/banner_top_img8.jpg", "img/banner_top_img9.jpg", "img/banner_top_img10.jpg", "img/banner_top_img11.jpg");

//da qui in poi non sono necessarie modifiche

if(document.getElementById) MakeHeader();

function MakeHeader(){

var el=document.getElementById(id);
if(el) el.style.background="url("+RandomImage()+")";
else setTimeout("MakeHeader()",100);
}

function RandomImage(){
return(imgArray[Math.round(Math.random()*(imgArray.length-1))]);
}