var banner = new Array();

banner[0]='http://i69.servimg.com/u/f69/11/79/36/51/30f90714.jpg';
banner[1]='http://i69.servimg.com/u/f69/11/79/36/51/abb00911.jpg';

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