Could it be that the paintings were based on real life locations in the Mushroom World?
I like that idea, the paintings being based on Mushroom locations.
.. and here is something else: I don't think that Peach has many castles (one for ceremonies, one for art, etc..) I think that the castle we see in every game is the same castle, just has been renovated, or magically changed.
SM64 saw that Bowser was trying to take over the Kingdom (again), this time by taking power from the castle, right? Well, when Mario defeated Bowser, what if the "evil" things that Bowser did disappeared? That would make the castle change it's shape/form to accomodate what has/had happened, and it's not like it's hard for Peach to save money for castle renovations when coins float around in the sky.
Speaking of coins, why do they float? I've never seen money that floats around like that.. and how come coins aren't just floating around in the RPG games (instead, held inside boxes or dropped by enemies)?