var banner = new Array();

banner[0]='http://i40.tinypic.com/35mlauc.png';
banner[1]='http://i43.tinypic.com/2cdu1rl.png';
banner[2]='http://i40.tinypic.com/ngr6u9.png';

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