var banner = new Array();

banner.push('http://img16.imageshack.us/img16/6/headern3.png');
banner.push('http://img815.imageshack.us/img815/978/headern2.png');
banner.push('http://img815.imageshack.us/img815/9537/headern1.png');

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)];",6000);