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.
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.
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.
You have to win the Pokemon contest by order. Starting with normal rank in Verdanturf Town then the super rank in Fallarbor Town and then hyper rank in Slateport City then finally master rank in Lilycove City. You have to enter one of your Pokemon in the cool contest, beauty contest, cute contest, smart contest or tough. after you win you will receive ribbens for the category you won. after wining master rank A painter will then paint your Pokemon and put it on display in the museum. For example i enterd my azumarill in the tough contest only in normal ,super, hyper and master ranks and i won then the painter that hangs around the berry maker next to the wall on the right in the master rank in lilycove told me my Pokemon picture was going to be on display in the museum.