That's a really
You could always go with a lot of the subjects of the actual historical photographs, like families or groups of friends, military units, etc. The limitations of the early process really influenced the early uses:
- Because of the expense and rarity, getting your picture taken was Kindof A Big Deal.™ Folks dressed in their finest, cleaned up as best they could (or were inclined) and limited picture taking to very special occasions. You see a lot of wedding photographs, birthday portraits, or still lifes that commemorate something important to a community: like the completion of building a church, town hall, or something else that was itself a Big Deal, like getting a railroad junction. Photographs of the dead were also quite common, as being dead made them excellent subjects, because...
- Exposure times were pretty long, so "candid" shots were obviously staged, action shots were unheard of and pictures of wildlife really only happened when said wildlife was quite dead. (Successful trophy hunt also falls squarely under the Big Deal stuff!) Pictures of a bustling main street, for example, really weren't feasible, because you had to get a fat load of people to stand still for a long time, and make sure anyone not supposed to be in the picture didn't just walk out on the main thoroughfare in the middle of the exposure. Pictures of empty streets (to show how developed the town was) were slightly easier, as you just had to keep folks out of the frame.
- "Artistic" photography was pretty rare indeed. Again, due to the expense involved, if it didn't have a commercial application it often went un-shot. Depending on how broad one wishes to classify art, pornography sells, but pictures of things like natural wonders were often limited to etchings because of printing limitations. Someone may commission a photograph of the Grand Canyon for their private collection, but if you were buying a travel guide, you were going to see an etching.
So depending on how you want to handle photography in your game, (perhaps only specific alchemical compounds work in most of the world that work much like the old photographs, as opposed to newer film) you could keep with a lot of these themes expanding into the story, or you could just have them represented as an "artistic choice" in the sort of photographs the NPC takes--a specific sort of self-limitation of subjects based on his aesthetic desires that happens to coincide with old-timey photos. Or just ignore the whole lot.
Rule of cool always wins.