While sneaking around in the school (after leaving the dorm and meeting up with Chloe), you'll find the science lab down a side hall (take a right at the intersecting halls, and look for either of the two science lab doors on the left). It's the same room you'll find the skeleton for the next picture. On the far wall, across from the doors, is a table with a small fish tank on it.
Look at it with Triangle, then turn on the light with X, and then take the photo with Square.
Any of the "optional photo" trophies can be easily obtained after finishing the episode by using chapter select and playing again in photo mode.