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.
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You have to win the master rank contest at lillycove city
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 deliver Devon's Goods to Captain Stern who is in Slateport City's museum.
If you win a Master Rank contest, a picture of your winning Pokemon will appear there. Cool contest - red frame, beauty contest - blue frame, and so on.
If you manage to get to Master rank Pokemon Contest, there will be a lot of people in the crowd, among them is an artist. When it's you're turn to show your Pokemon to the crowd, make sure there a heart pops up over his head, because that means if you win, your Pokemon will be drawn in full color, and will be placed in Lilycove Museum, making you a "super sketch model."