In Cascade Kingdom, there is an empty portrait hidden behind the waterfall. When walking up to it, Cappy wonders to himself what the portrait might be used for.

I later found a second empty portrait in Sand Kingdom on the side of a tower.

What do these empty portraits do? Is there some way to interact with them?

2 Answers

As far as I can tell, portraits are warps to other worlds, usually to snag a power moon. At this time I'm not sure what activates a portrait (previously I thought you had to beat or visit a world to unlock it).

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They are hidden portraits, which are portals to a hidden place in the game.

For the hidden portraits to unlock you usually need to beat the Kingdom Boss, or at least that's what these two articles say.

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