Throughout the game you will come across painting on the walls dedicated to given characters. Their appearance depends on the combination of characters that you choose and can vary between playthroughs. Most paintings are visible and accessible during normal exploration, but some finding and reaching some of them can be troublesome.
The painting can be found behind the old well at the back of the cave, in the employee only corridor. It can be accessed after using rank on the well and getting the bucket to the surface.
The painting can be found in one of the castle chambers which you can access with the lift lowered by another character.
The painting is on a wall in the museum, where a dinosaur was before in the past.
The painting is on a island in the lava pit. You can get there using the adventurer's special skill.
The painting is in The Living Room of the twin's house, behind a bookshelf. You need to click on it a couple times to knock it over.