var banner = new Array();

banner[0]='https://i.servimg.com/u/f40/14/00/16/76/dulce_14.jpg';
banner[1]='https://i.servimg.com/u/f40/14/00/16/76/dulce_14.jpg';

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)];",5000);