I found this old forum post (from 5 years ago) and was wondering if anything similar has been implemented?
http://www.zenphoto.org/support/topic.php?id=2604

Since we aren't adding user accounts to our system, we would like the ability for guests to add favorite photos, and have those persist through each session. As well as be able to see within the admin panel.

Not with our favorites feature. I would suggest to use the rating plugin instead.