The instructions below will help you to know that how you can change a wallpaper on your BlackBerry PlayBook tablet.
Change Wallpaper on BlackBerry PlayBook
Open “Pictures” app on PlayBook
Open pictures album and tap the picture that you want to set as a Wallpaper
Close the pictures apps, go back to the home screen and see the wallpaper is changed
If you want to set a wallpaper that is available in your computer then see how to transfer files to PlayBook from PC