var banner = new Array();

banner[0]='http://i56.tinypic.com/34ytmb8.jpg';
banner[1]='http://i55.tinypic.com/ilc1vk.jpg';
banner[2]='http://i52.tinypic.com/30bpjco.jpg';

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