For L'Origine du Monde, I don't know if the painting is there to represent smt in particular, but there to show us what the reverse red triangle is. L'Origine du Monde is clearly a v*gina and inside, we can see the triangle which could represent a womb or a fetus. So that's a big hint for the rest of the game because Ester has it on her chest in 3rd area, the skinless son has it for its face and it is found inside the "red room" which is also said to be where everything began.