var banner = new Array();

banner.push('https://i.servimg.com/u/f61/16/01/38/12/bannie11.jpg');
banner.push('http://jolstatic.fr/www/captures/1012/4/18634.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);