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.
Best Answer
There are two kinds of paintings in the game. One you can encounter "in the wild"; they lead to various kingdoms; the others lead you to a "sub-level", or such.
Some of the portal paintings are filled regardless of what you do; they usually let you have a peak at some kingdom you've yet to visit, or bring you to the one you've already been to.
The unfilled paintings are mostly portal paintings which you've not unlocked yet. They usually lead to kingdoms which you're not supposed to see yet, such as the one in the Cascade Kingdom; they unlock only when you've reached the other kingdom by normal means.
The moon church painting is not a portal painting, it is similar to the one right in front of it, and is the only blank painting of it's kind; it unlocks when you collect all non-shop moons, i.e. 880 of them.