From Bulbapedia, the community-driven Pokémon encyclopedia.
Backdrops (Japanese: はいけい Background) are items introduced in Generation IV. They are stored in the Fashion Case, along with Accessories.
They are primarily used in Super Contests, but can also be used on the second floor of the Jubilife TV building. Only one can be used at a time. Players of Diamond and Pearl can win more daily in the Pokémon Lottery Corner draw, while players of Platinum can get a random one once a day from a woman at the Global Terminal in Jubilife City. Players of HeartGold and SoulSilver can randomly receive Backdrops when entering a drawing that a man in the Goldenrod Tunnel runs, but the chances of getting one decrease the more the player wins. They can also be obtained from a lady in Pal Park who is watching the Catching Show.
List of Backdrops