haven't played for a while but i think you buy them from the bloke in the tent, who comes oncea week and you need a password to get in. Some times they are fakes but if they're not you can give them to the museum.
hm every time i go to the tent and buy a painting i take it to the museum the owl always says it is a fake and that he could not accept it. I think you have to get insurance as you have to pay 3.000 bells for the pitcure to just be told its a fake and have to sell it at the shop for 10 bells, Ridiculous! But thankyou i think i will keep buying them till i get a real one!!! :)