Which URL? I depends on if there is an API which you can hook into. From my knowledge it is not something you would be able to do ‘out-of-the-box’. It would require you to make some php script that fetched the images. If you want to display the pictures from an external site on your own, then I dont quite understand why you want NGG to play a part in that?
Check ngg-db.php line 612.
function add_image( $id = false, $filename = false, $description = ”, $alttext = ”, $meta_data = false, $post_id = 0, $imagedate = ‘0000-00-00 00:00:00’, $exclude = 0, $sortorder = 0 )
This is how you in PHP can add images to the ngg database.