This query looks fine for getting data from 1 table:
$sql = "SELECT image_id, COUNT(*) FROM download_registratie GROUP BY image_id";
$result = mysql_query($sql);
But if I'm reading your problem correctly you want to get the image name from a second table. This should do the trick:
SELECT dr.image_id, di.name, count(dr.image_id) AS cnt
FROM download_registratie dr, download_image di
WHERE dr.image_id = di.id
GROUP BY image_id
ORDER BY cnt DESC