Instructions
Turn on the PS3, then select the username that you use to log in to your system.
Scroll through the menu using the left and right arrows on your controller and select the "Settings" option.
Use the up and down arrows on the controller to navigate to the "Theme Settings" option, then press the "X" button to select it.
Select the "Theme" option by pressing the "X" button.
Scroll through your installed themes until you locate the "Original" or "Classic" theme. These are both installed by default on your PS3.
Press the "X" button on your selected theme to change the wallpaper on your PS3 menu. These themes adjust automatically and will periodically change your wallpaper.