function SimpleSwap(el,which){ el.src=el.getAttribute(which || "origsrc"); } 
function SimpleSwapSetup(){ 
var x = document.getElementsByTagName("img"); 
for (var i=0;i<x.length;i++){
var oversrc = x[i].getAttribute("oversrc"); 
if (!oversrc) continue;
// comment the next two lines to disable image pre-loading 
x[i].oversrc_img = new Image(); 
x[i].oversrc_img.src=oversrc;
// set event handlers 
x[i].onmouseover = new Function("SimpleSwap(this,'oversrc');"); 
x[i].onmouseout = new Function("SimpleSwap(this);"); 
// save original src
 x[i].setAttribute("origsrc",x[i].src);
 
 }
 } 
 var PreSimpleSwapOnload =(window.onload)? window.onload : function(){}; window.onload = function(){PreSimpleSwapOnload(); SimpleSwapSetup(); setContent();}

//Read more: http://www.webstylepress.com/simple-rollover-swap-image/#ixzz1aCNgDLXj