I believe that you get them from winning master-rank contest (1for each master contest) in fact if you get all the paintings and donate them to the muesam in lilycove city i think you get an ICE SCULPTURE, basically a big Articuno which is a legendary bird poke'mon from Kanto, any way it will look good in your secret-base.
That one is pretty simple. You have to win the master rank contests of Beauty, Cute, Cool, Smart and Tough twice. In your second attempt, you have to make sure that your heart meter is full or otherwise, you won't inspire the artist that will paint the Pokemon. On your second victory, if you had a full heart meter and won the contest, (The artist doesn't need to have a heart above his head for you to get a painting) the artist will come out and say that he was inspired to paint better than usual, these paintings will be displayed in the museum if you said yes. (There is one Pokemon that can be in all paintings, my Rayquaza, Jupiter.)
OK so first you will need to win the Master Rank of the Pokemon Contest to one Pokemon then talk to the painter and it will be in the museum.
In the Pokemon series you can revive fossils into rare Pokemon. In Pearl you can do this at the coal museum.
go in the pewter city museum of science then talk to the person close to the entrance(not outside the museum)
Take it to the museum in Nacrene City
i think you have to give the fossils to the museum in pewter city
If your Pokemon scores five hearts or more in a contest, you'll get a portrait of it that will go to the Lilycove Museum.
It's in Lilycove City.
When you win first place in a Master Rank Pokemon contest, and do exceptionally well, there is a chance that the artist outside of the contest hall will show you a portrait of your winning Pokemon that he was inspired to make because of their (cute-/smart-/etc.)-ness, and will ask you if you want it displayed in the Art Museum. If you agree, then the portrait will be displayed.
You have to win the master rank contest at lillycove city
OK so first you will need to win the Master Rank of the Pokemon Contest to one Pokemon then talk to the painter and it will be in the museum.
The art muesum is located at Lilycove City, at the top left.
Win inthe Master Class Contest then talk to the guy in lilycove museum :-)
While in Lilycove City, you can find Captain Stern in that museum place where Team Magma/Aqua are standing by. I forgot how to get rid of them, but once you do, you can find Captain Stern in that museum place I mentioned.
You must win a Master Rank Contest by a certain number of points above the others, then the artist will paint your Pokemon and put it in the Lilycove Museum.
There's an artist who sketches the winner of the contests in each contest hall. When you win the Master rank, he will do a more detailed one. He will ask you if he should take it to the museum, so tell him yes. He'll take it there. (It doesn't take him very long since that contest hall and the museum are in the same town). If you go to the upper level of the museum, you can view the picture.
In the Lilycove City, if 5 paintings of your Pokemon who won the contest are exhibited to the Art Gallery's second floor, the Curator will give you a Statue of a Legendary Pokemon... But about the Art Dealer outside, I still don't know what to do with him...
He's in the museum.