There is a special feature in Pokémon Emerald which allows you to get a special Box Wallpaper. You need to be a member of the Pokémon Fan Club and go to their website, input your Trainer ID and you'll get a name. Go to this house in Rustboro and go to the top floor
Talk to the guy standing near his Daughter & Her PokéDolls and he will give you the chance to fill in a code
Fill it in Correctly and he'll tell his Daughter and then talk to you again and he'll tell you about the new Box Paper
Go to the Box and select the new option. There is only one known Paper as of yet, When more are known, they'll be added here

 
All Content is ©Copyright of Serebii.net 1999-2013.
Pokémon And All Respective Names are Trademark & © of Nintendo 1996-2013