You could use clear epoxy resin to coat the photos.
The problem with epoxy resin is that it yellows over time. There is
one, called Hxtal-Nyl, that yellows very minimally and very slowly.
It was developed for glass restoration and has optical properties
that are similar to glass. Unfortunately, it is pricey and many
people find it hard to work with. If you want to try it, I’d be
glad to give you some tips. (You can contact me off-list.)
Dale Molloy
Molloy Studios
Fall River, MA