var banner = new Array();

banner[0]='http://i43.tinypic.com/2vn59jb.jpg_1';
banner[1]='http://i42.tinypic.com/2nqvfbp.jpg_2';
banner[2]='http://i43.tinypic.com/x3fibl.jpg_3';
banner[3]='http://i40.tinypic.com/206zn9l.jpg_4';

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