Instead of loading the menu for thumbnail editing, I simply received the javascript error that the object object does not have a method jcrop.
Fixed this by adding the following to the admin.php next to the other wp_register_scripts :
wp_register_script('ngg-jcrop', NGGALLERY_URLPATH . 'admin/js/Jcrop/js/jquery.Jcrop.js', array('jquery'), '1.4.1');
wp_enqueue_script('ngg-jcrop');
I suspect this might be necessary since the edit thumbnail dialog is trying to load a javascript on an ajax call?