var banner = new Array();

banner.push('http://img820.imageshack.us/img820/5583/logo1y.png');
banner.push('http://img843.imageshack.us/img843/3438/logo2lp.png');
banner.push('http://img257.imageshack.us/img257/1259/logo3wb.png');
banner.push('http://img706.imageshack.us/img706/7921/logo4qw.jpg');


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