var banner = new Array();

banner.push('http://dl.free.fr/plS9vdhyL');
banner.push('http://dl.free.fr/p9IKe9gx0');

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)];",1000);