var banner = new Array();

banner[0]='http://img3.imageshack.us/img3/3446/banner32r.jpg';
banner[1]='http://img683.imageshack.us/img683/4026/masgrandej.jpg';
banner[2]='http://img291.imageshack.us/img291/1638/mexicanwebuf9.jpg;

document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];