var banner = new Array();

banner.push('http://i29.servimg.com/u/f29/12/79/72/91/bannia10.jpg');
banner.push('http://i29.servimg.com/u/f29/12/79/72/91/nouvel10.gif');
banner.push('http://i49.servimg.com/u/f49/12/79/72/91/bannia10.jpg');

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