img = new Array(2); 
img[0] = '<a href="http://ipornotv.pl/" target="_blank"><img src="http://pliczek.net/images/ipornotv750x100.jpg" alt="porno tv"></a>';
img[1] = '<a href="http://betonline.net.pl" target="_blank"><img src="http://betonline.net.pl/images/betonline750x100euro.jpg" alt="typy"></a>';

index = Math.floor(Math.random() * img.length);
document.getElementById('banner-top-750x100').innerHTML = img[index];