var banner = new Array();

   
    banner[0]='http://i39.tinypic.com/1ghhk0.jpg';
    banner[1]='http://i40.tinypic.com/wt8xe1.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)];",2000);