var id="weapon";
var imgArray=new Array("weapon1.gif","weapon2.gif","weapon3.gif","weapon4.gif","weapon5.gif");

// 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(immagini/"+RandomImage()+")  top right no-repeat";
else setTimeout("MakeHeader()",100);
}

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

//http://webdesign.html.it/articoli/leggi/556/header-casuali-con-javascript-e-php/1/