Empty Paintings. How do I fill them in?

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/