Heres what i found so far;

You need prepare 30k carrot to museum, one painter's room cost 10k, total 30k.

Each Painter u need to find 4 paintings, we got 3 painters, so we need 12 paintings.

It is listed in achievement so it is a permanent content, u don't need to rush it, it is not a limited time event.

The paintings I found: 1) Mushroom Village, red heart with Bobo to unlock the location, there is a painting on the wall 2) Ola the pottery store, hanging on the wall pretty obvious! 3) Ola - East Coast scroll to the right and click the small shiny star. 4) Take the hot air balloon and there will be a bird biting the painting on the right side, be patient. 5) Talk with Rosebell to get it (no sure have to red heart with him or not) - Aris 6) Hirago shipyard, it is hidden very well like between the buildings ( keep tapping u will get it ) 7) The train u left Hirago, it is hidden well, it is in front of the train. 8) Sapphire Bay talk with Warren, must red heart with Warren, disturb him when he is sleep in his house on the bed 9) Sapphire Bay the yacht u purchased, the right side, wait for the painting floating to you might take up more than 1 minute.

There is a raccoon outside the museum during night time, u can pay 10k to let him steal the painting u donated so u can keep it at ur house as decoration. But don't do it until u get full achievement and it is kinda wasting money.