$(document).ready(function(){
   // Your code here
  $(".nav > li > a > img").mouseover(function(){
		filename = this.src;
		filename = filename.replace(".gif","-roll.gif");
		this.src = filename;
  });
 
	$(".nav > li > a > img").mouseout(function(){
		filename = this.src;
		filename = filename.replace("-roll.gif",".gif");
		this.src = filename;
		});
	
 });