var banner = new Array();
 
banner.push('https://nsa21.casimages.com/img/2012/02/06/120206123258583400.png');
banner.push('https://nsa22.casimages.com/img/2012/02/06/120206123345340097.png');
banner.push('https://nsm07.casimages.com/img/2012/02/06/1202061256301249669399921.png');

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