var id="header";
var imgArray=new Array("header1.jpg","header2.jpg","header3.jpg","header4.jpg","header5.jpg","header6.jpg","header7.jpg");

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))]);
}