var banner = new Array();

banner.push('http://img824.imageshack.us/img824/7624/headerabk.png');
banner.push('http://img199.imageshack.us/img199/9798/headercre.png');
banner.push('http://img828.imageshack.us/img828/403/headerchq.png');

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