Like paintings, I found that photo values appreciate slowly over time (much slower than bottled nectar). Their value depends on the subject photographed, the skill level of the sim taking the picture, what photo size & color (ie color panoramic or sepia tone v small black & white photo), and how many times that particular subject has been photographed. I think the appreciation rate may be similar to the way nectar works. Ie the photo appreciation rate may have something to do with whether the photos are in a sims inventory or displayed on a wall. I've only done the former, so this needs to be confirmed
Best way to max the money in this slow burn freelance career, is to level your sim to lvl 10 around your base hood. Have them take a variety of different subjects around the hood. Try to complete the photo challenges in their skill book and take repeat photos of same subject if necessary. Sell the lower value photos and begin an art collection of the larger more valued ones. Create a room in their house and store the more valuable pieces on the walls.
Once your sim is at lvl10, send them to a WA world. This is because the photo challenge is hard coded for WA worlds in the skill book. So you need to take pics here to get credit for the photo challenge. Have your sim take panoramic color shots of all the famous WA landmarks like the Sphinx, etc. Make sure this is the very first time they're photographing these. Unique subjects, skill level, and number times a subject is taken is what drives the photo value.
And yes, photos apparently continue to appreciate once the sim dies. Just like how paintings do once a sim dies and in irl lol. So keep your sim diligently collecting a priceless collection of WA and other first time panoramic shots in custom worlds.