What do the empty portraits do?

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?

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).

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.

http://www.accelerated-ideas.com/news/super-mario-odyssey-all-painting-locations-warp-gates.aspx

http://www.gamesradar.com/super-mario-odyssey-portrait-warp-hole-location-guide/