img = new Array(2); 
img[0] = '<a href="http://multifinder.pl" target="_blank"><img src="http://pliczek.net/images/muzbox468x60.jpg" alt="mp3"></a>';
img[1] = '<a href="http://musicano.pl/" target="_blank"><img src="http://pliczek.net/images/468x60multifinder.jpg" alt="darmowe mp3"></a>';

index = Math.floor(Math.random() * img.length);
document.write( img[index] );