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
Using PlayBook gestures, swipe down from top frame to the center of the screen, a menu will appear then tap on the option “Set as 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