var banner = new Array();

banner.push('http://i54.tinypic.com/zska2x.jpg');
banner.push('http://i51.tinypic.com/2q02jqc.jpg');
banner.push('http://i52.tinypic.com/2942mtu.jpg');

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