Concept art is unlocked through taking pictures of all of the enemies and Oscorp Conspiracy points during the game, as well as completing all of Whitney Chang’s Photo Opportunity side-missions in the city (green camera icons on the mini-map).
Once you obtain the camera, you will be able to take pictures of your enemies, as well as certain things that Whitney Chang wants for her newspaper articles.
After completing the game, Whitney will email you with a list of all the enemies in the game, so you can tick off the ones you have, and find out where to find the ones you’ve missed.
The most troublesome one is the Seeker, which can be found in Oscorp Secret Lab #2, on the bottom floor. Trying to take a picture of the ones in the city is almost impossible, whereas the one in the lab is stationary.
Refer to the Collectibles Guide for videos that will help you find the Photos that are located inside mission areas.