var banner = new Array();

banner[0]='https://i.servimg.com/u/f43/14/93/78/66/banner17.png';   
banner[1]='https://i.servimg.com/u/f43/14/93/78/66/banner11.png';

    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)];",5000);