var banner = new Array();

banner[0]='http://img840.imageshack.us/img840/3844/neepaotitulo.png';
banner[1]='http://img684.imageshack.us/img684/7711/fondoparaforo.png';
banner[2]='http://img857.imageshack.us/img857/1795/asddddddddddddd23.png';

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