var banner = new Array();

banner[0]='http://i56.tinypic.com/330fhqd.png';
banner[1]='http://i56.tinypic.com/1q1snc.png';
banner[2]='http://i53.tinypic.com/5ma5fl.png';
banner[3]='http://i55.tinypic.com/vdctjm.png';

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