banner[0]='http://img413.imageshack.us/img413/2892/primero.jpg';
banner[1]='http://img69.imageshack.us/img69/7458/segunday.jpg';
banner[2]='http://img190.imageshack.us/img190/8688/tercera.jpg';

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

setInterval("document.getElementById('i_logo').src=banner[Math.floor(Math.random()*banner.length)];",3000);