Like the iPhone, you can use a custom wallpaper. First, use Safari to find the image you want. (Try Interfacelift.com, which has iPad-sized images.)
View the image, then press and hold on the screen and select Save Image. Now, go to Settings, Brightness & Wallpaper. Flick to the right, go to Saved Photos, and select the image. Press the Set Lock Screen or Set Home Screen option, or Both.