img = new Array(2); 
img[0] = '<a href="http://www.ligipolskie.pl/" target="_blank"><img src="http://betonline.net.pl/images/ligipolskie_750x100.jpg" alt="sport"></a>';
img[1] = '<a href="http://topmp3.pl/" target="_blank"><img src="http://pliczek.net/images/topmp3_750x100.gif" alt="mp3"></a>';

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