var banner = new Array();

banner[0]='http://img210.imageshack.us/img210/2269/ban41.jpg';
banner[1]='http://img115.imageshack.us/img115/9800/ban42.jpg';
banner[2]='http://img267.imageshack.us/img267/7195/ban43.jpg';

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