There doesn't seem to be a way to hide spoilers on Itch.io, so I'm going to put a big spoiler warning here for other players...
SPOILERS AHEAD
If you ask Shirley for a hint about the paintings, she says something like, "A couple of the places you should examine are right outside the manor. You should also check out the tailor's shop in the village. And the last area is near the farm." (The exact wording depends on whether you've unlocked the tailor and farmer, but sounds like you have.)
So you need to have four paintings (Rudolph's painting is NOT one of them), in order to examine the four locations mentioned in her hint: two right outside the manor, one in the tailor's shop, and the last area near the farm. Good luck!