var banner = new Array();

banner[0]='http://a.imageshack.us/img823/3096/buizeltry1.png';
banner[1]='http://a.imageshack.us/img22/2750/megamann.png';

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);