var banner = new Array();

banner[0]='http://img195.imageshack.us/img195/1999/logouma.jpg';
banner[1]='http://img35.imageshack.us/img35/2992/logodois.jpg';
banner[2]='http://img821.imageshack.us/img821/4743/logotrs.jpg';
banner[3]='http://img689.imageshack.us/img689/2602/logoquatro.jpg';
banner[4]='http://img69.imageshack.us/img69/959/logocinco.jpg';

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