var banner = new Array();

banner[0]='https://i.servimg.com/u/f42/15/62/24/21/logo10.gif';
banner[1]='https://i.servimg.com/u/f42/15/62/24/21/logo210.gif';
banner[2]='https://i.servimg.com/u/f42/15/62/24/21/logo310.gif';
banner[3]='https://i.servimg.com/u/f42/15/62/24/21/logo410.gif';

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