var banner = new Array();

    banner[0]='https://i.imgur.com/SOdlSiB.png';
    banner[1]='https://i.imgur.com/ZkHMZws.png';
    banner[2]='https://i.imgur.com/6dCMZd7.png';
    banner[3]='https://i.imgur.com/YnMeJey.png';

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