var banner = new Array();

    banner[0]='http://i23.servimg.com/u/f23/16/53/92/35/head10.jpg';
    banner[1]='http://i.imgur.com/C1PR8.jpg';
    banner[2]='http://i23.servimg.com/u/f23/16/53/92/35/head10.jpg';

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