Don't know if there is a policy about tagging the copyright when it comes to Pokémon Games.
So the way I'd suggest is (and I've tagged it this way for now):
Tag it with the game the Pokémon first appeared in.
That happens right now with the new Pokémon that will be appearing the first time in Pokémon SM.
I.e. every Rowlet pic should get tagged with pokemon_sm
Every pic where Pikachu is appearing should get tagged with pokemon_rgby since it first appeared in those editions.