var banner = new Array();

banner[0]='http://lh4.ggpht.com/_uaBLe1y2iWY/Sxzytg6Nc6I/AAAAAAAAAc8/aouXYp8EGCk/Cabecera%20Definitiva%20Posada1.png';
banner[1]='http://lh4.ggpht.com/_uaBLe1y2iWY/Sxzytg6Nc6I/AAAAAAAAAc8/aouXYp8EGCk/s912/Cabecera%20Definitiva%20Posada1.png';
banner[2]='http://lh4.ggpht.com/_uaBLe1y2iWY/Sxzytg6Nc6I/AAAAAAAAAc8/aouXYp8EGCk/Cabecera%20Definitiva%20Posada1.png';

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