Lately I've been going down the Luigi's mansion rabbithole and I've been thinking of King Boo's powers.
So we all know that he's able to create portraits to capture people right?
But you know who else has used paintings to do evil things?
Bowser in Super Mario 64/64 DS. In there, Toad said that Bowser "stole the power stars and used their magic to create evil lands of monsters in the walls and paintings."
So because of this, I have a small headcannon/theory that King Boo took inspiration from Bowser's use of the paintings in Mario 64.
I mean he had to get the inspiration to use paintings somewhere. (Imo)
Real fast, whether you believe 64 DS or the OG 64 is canon, I like to think that either way, King Boo took notice of what Bowser did and wanted to build off of it.
I haven't played the first Luigi's Mansion, but if I recall correctly, King Boo used E. Gadd's portraitficationizer just in reverse to free the ghosts and Boolossus. Then he used it to trap Mario in a painting, however Mario could still move in his portrait prison (somehow).
So he didn't do anything himself there, but by the time of Dark moon, we see him actually learn to create portraits via his new crown magic and make them still so his victims can't move or yell for help in them.
Then by Luigi's Mansion 3, King Boo has basically perfected the art of prison portraits (pun intended) and even learned how to put more victims in them and to increase their size for larger traps.
So it seems through his experimentation with them, he developed his portrait powers.
TLDR: King Boo liked what Boswer did with paintings and just rolled with it, just in a completely more deranged way and his use of them evolved over time.
IDK, I just noticed that they've both used paintings before then this headcannon/theory formed. Hope you liked my random rambling about my 2 favorite Nintendo villains.
Also, I'm sure this isn't the most well thought out, but still, it's kinda interesting to think about.