var banner = new Array();

banner[0]='http://i44.tinypic.com/287lll5.gi';
banner[1]='http://s3.amazonaws.com/data.tumblr.com/tumblr_lyldznAurH1r2i7obo1_1280.gif';

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);