var banner = new Array();

banner[0]='http://i13.servimg.com/u/f13/11/63/48/55/portal10.jpg';
banner[1]='http://i83.servimg.com/u/f83/11/63/48/55/48532711.jpg';
banner[2]='http://i642.photobucket.com/albums/uu149/FMBB_album/71206810.jpg';
banner[3]='http://img32.imageshack.us/img32/1091/86232720.jpg';
banner[4]='http://img697.imageshack.us/img697/8472/74028813451976048403209.jpg';

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