var banner = new Array();

banner[0]='https://i.servimg.com/u/f35/15/05/40/97/1logo10.png';
banner[1]='https://i.servimg.com/u/f35/15/05/40/97/2logo10.png';
banner[2]='https://i.servimg.com/u/f35/15/05/40/97/2logo10.png';

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