var banner = new Array();

banner[0]='https://i.imgur.com/owPynId.png';
banner[1]='https://i.imgur.com/L350elB.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);