var banner = new Array();

banner.push('http://img59.imageshack.us/img59/2649/bannire2l.jpg');
banner.push('http://img180.imageshack.us/img180/7822/bannire42.png');
banner.push('https://i.servimg.com/u/f61/13/31/10/77/banncl11.jpg');
banner.push('http://img339.imageshack.us/img339/6365/bannireclbisbis.png');

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