I came across, during game play, some paintings. Most were with a painting inside, and you could jump through, Mario 64 style.
But I came across some empty paintings. I expect that I can unlock them, fill them in, and have access to a new boss/world through them.
For instance, in the moon church, there's a bowser painting, but in front of it there's an empty painting.
So How do I fill the empty paintings?
(Sorry if Paintings is not that descriptive, English is not my first language and I can't find what the damn word for the support around the painting.)
PS: Other question asks what they do, I ask how to unlock them. So yes, Heavily related; but No, not duplicate.