var banner = new Array();

    banner[0]='http://i.imgur.com/qjufE.jpg';
    banner[1]='http://i.imgur.com/rpcph.jpg';
    banner[2]='http://i.imgur.com/7OBxY.jpg';
    banner[3]='http://i.imgur.com/trY5q.jpg';

    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)];",3000);