The human eye forms an image on the retina - the light sensitive part - and transfers information about the image to the brain. Just like film in a camera, this process involves reactions. The light falling on the retina stimulates nerves. It takes time for the stimulation to clear. Normally we don't look at one thing for a long period, so further stimulation happens, over-riding the last image about 20 times a second.
If we stare at things, however, things are different - especially when what we stare at has dark and light areas (as in the example here). Dark regions have little (or no) light coming from them - so no stimulation occurs. The eye grows used to the image it sees, and the nerves go on relaying the same information, over and over.
In the, initially strange, example here the brain is confused by what it sees, and tries to add detail to it. Only when you close your eyes (zero stimulation) does the fading image of Jesus of Nazareth "appear". Unexpected is the fact that his hair is dark - whereas in the image, it's bright! This is the result of over-stimulation. Some people imagine they see a colour image - in this case the result of their brain over-actively filling in missing detail.