var banner = new Array();

banner[0]='http://img840.imageshack.us/img840/803/bannernavidad.png';
banner[1]='http://img41.imageshack.us/img41/8509/bannernavidad2.png';
banner[2]='http://img683.imageshack.us/img683/8661/bannernavidad3.png';

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

setInterval("document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];",10000);