// JavaScript Document

arr_img=new Array();
arr_img[0]='ad1.jpg';
arr_img[1]='ad2.jpg';
arr_img[2]='ad3.jpg';
arr_img[3]='ad4.jpg';

function randomBkg() {
	var str_img = "images/"+arr_img[Math.round(Math.random()*3)];
	document.getElementById("mainImage").setAttribute("src", str_img);
		}