This is not a problem. The iPad screens are made with LED screens, which do not get damaged from staying on long times.
Old CRT screens had a problem that images could "burn" into the screen. This does not happen with LED screens.
However, it is good to "exercise" LED crystals regularly. That's why we designed the Photo Display app to rotate between a number of photos.
Also, the app has a "Dark Screen" feature, where you can darken the screen during certain hours. This is useful if the screen light is disturbing your sleep or lifestyle.