I've enabled the heart icon for favorites when viewing an image in lightbox mode.

The heart icon on the right of the lightbox is to "favorite" a photo. If a user has already marked a photo as a favorite, it will appear in red - clicking a red heart icon will un-favorite the image.

Favorites by anonymous visitors are counted - but they can't favorite the same image multiple times.

Next up is showing the heart icon on images within the album view, as well as the number of favorites.