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.
75 Answers
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.
1For the hidden portraits to unlock you usually need to beat the Kingdom Boss, or at least that's what these two articles say.
2From my experience, all the other answers are wrong, and it's much simpler than they are claiming.
Only some paintings are locked, and they all unlock after you rescue Peach.
This excludes the Boss Rematch paintings, which require you to have beaten the boss in the first place (and in at least one case, have enough moons).
5Spoilers ahead, but then again, this game is six months old already.
Depending on the Kingdom, there seems to be one of two unlock triggers.
Some Kingdoms have their paintings unlocked when the boss is defeated (that is to say, the boss in the Kingdom you’re leaving, not the destination Kingdom). In the event that there’s multiple bosses, some Kingdoms pick on the first boss (like Sand), and some Kingdoms pick on the second (like Luncheon). Other Kingdoms that function like this include Lake, Wooded, Metro, Seaside, and Bowser’s.
Other Kingdoms have their paintings unlocked once you’ve been to the other side. While usually this is to prevent late-game spoilers (such as Cascade’s not being filled in until you’ve beaten Bowser’s Kingdom), sometimes this means that the painting is already filled in once you get there. Other Kingdoms that function this way include Snow and Mushroom. (As an aside, I’m not sure why the Luncheon painting doesn’t work the same way, since it spoils Mushroom Kingdom.)
The one on the Moon is different. As others have mentioned previously, that painting unlocks a harder Bowser fight once all 880 main moons have been collected.
4The paintings fill in gradually as soon as you find the "other side" - Those are usually not accesibble without bugs before they are filled. Others fill when you progress. Those are really the two types of painitngs in that game.
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