var id="header";
var imgArray=new Array("1.jpg","2.jpg","3.jpg",
  "4.jpg","6.jpg","9.jpg","11.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))]);
}