Hi Real,
Did you consider using the database activity module instead of the assignment module? You can start with separate groups if you want to keep all images hidden from other students and change the set up after grading. Grading can be done in an offline assignment. In the database activity module students could also comment on other images if you like this. In the database for databases I found "imagegallery_with_rss_comment_grades."
With kind regards,
Hartmut