var banner = new Array();

banner[0]='http://i84.photobucket.com/albums/k30/as3051048/UN_LOGO_AZUL.png';
banner[1]='http://i84.photobucket.com/albums/k30/as3051048/UN_LOGO_VERDE.png';
banner[2]='http://i84.photobucket.com/albums/k30/as3051048/UN_LOGO_LARANJA.png';
banner[3]='http://i84.photobucket.com/albums/k30/as3051048/UN_LOGO_VERMELHO.png';

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