var banner = new Array();

banner[0]='http://img33.imageshack.us/img33/968/34328416.jpg';
banner[1]='http://img194.imageshack.us/img194/4228/54096621.jpg';
banner[2]='http://img69.imageshack.us/img69/9162/16661771.jpg';

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