Instructions
Go to Rustboro City and speak to the old man with the sick daughter. He will ask you for funny words to make his daughter laugh. Either enter words at random until she laughs or visit a code generating website. (See Resources.)
Enter your "Trainer ID Number" into the correct box on the website. The trainer ID number is located on the top right-hand corner of your trainer card, which comes with the game. Enter any other information that the page asks for.
Choose among the options the page gives you for the design of the wallpaper that you want. You can change "Background Pattern" and "Icons" by selecting from the drop-down menus, and pick a "Background Color" and "Foreground Color" by entering the correct code numbers. Click on "Help" beside each box for a list of the available colors and their codes.
Click on "Generate Secret Word" at the bottom of the screen when you are happy with your wallpaper design. Copy this word down and go back to the Pokemon game.
Go through the dialogue with the old man until he asks you for a word. Enter the secret word from the website. The girl will laugh, and your wallpaper will appear.