var banner = new Array();

banner[0]='http://imageshack.us/a/img194/9837/52441241.png';
banner[1]='http://imageshack.us/a/img541/5137/htrhrth.png';
banner[2]='url_imagen_3';

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