While sneaking through the school (after leaving the dorms and meeting up with Chloe), you'll be able to go down a side hall and into the science lab (you may remember it from Episode 2). At the back of the room, beside a couple of cabinets (the weed killer is on top of one of them), there is a skeleton. Photo the skeleton.
Any of the "optional photo" trophies can be easily obtained after finishing the episode by using chapter select and playing again in photo mode.