var banner = new Array();

    banner[0]='http://i.imgur.com/NFJzE.png';
    banner[1]='http://i.imgur.com/553fo.png';
    banner[2]='http://i.imgur.com/7zCp7.jpg';    
    banner[3]='http://i.imgur.com/lPcq4.png';    
    banner[4]='http://i.imgur.com/4TP2y.png';    
    banner[5]='http://i.imgur.com/aNSf5.png';    

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