var banner = new Array();

banner[0]='http://img6.imageshack.us/img6/5017/geobanerlogo2.png';
banner[1]='http://img6.imageshack.us/img6/9742/geobanerlogo3.png';
banner[2]='http://i17.servimg.com/u/f17/11/47/76/77/geoban10.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)];",30000);