var banner = new Array();

banner.push('https://i.servimg.com/u/f63/14/83/02/63/bannie11.jpg');
banner.push('https://i.servimg.com/u/f63/14/83/02/63/bannie11.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)];",3000);