var banner = new Array();

banner.push('https://i.servimg.com/u/f36/17/66/98/61/bancoa10.jpg');
banner.push('https://i.servimg.com/u/f36/17/66/98/61/bancoa11.jpg');
banner.push('https://i.servimg.com/u/f36/17/66/98/61/bancoa12.jpg');

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