var banner = new Array();

banner[0]='http://img689.imageshack.us/img689/8883/88217392.png';
banner[1]='http://img706.imageshack.us/img706/7230/bannerwv.jpg';
banner[2]='http://img508.imageshack.us/img508/1315/yamagoku.jpg';

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