There is a useful article on the Apple support site regarding "image persistence" on Apple LCD displays.

Commonly referred to as screen burn, this used to be a major problem with CRTs that displayed the same image for a long while, and it got "burnt" into the screen so that it was permanently displayed.

Many people think that LCD displays are unable to get screen burn, but actually they can. Manufacturers call these faint remnants of an image "persistence". However it is quite easily solved, as detailed in the Apple article.

To avoid screen burn, set the energy saver preferences to turn off the display when not in use. To try and remove persistence, create a slideshow screensaver with one white photo, and display it until the persistence is gone.