var banner = new Array();

banner[0]='http://imageshack.us/a/img16/4730/banner1zj.jpg';
banner[1]='http://img29.imageshack.us/img29/9964/banner2od.jpg';

document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];
setInterval("document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];",29000);