var banner = new Array();

banner[0]='http://img11.imageshack.us/img11/8671/headerbsg.png';
banner[1]='http://img857.imageshack.us/img857/1739/headerbsgv3.png';
banner[2]='http://img10.imageshack.us/img10/7043/headerbsgv4.png';

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