How does your eye sustain a picture, after your eyes adjust in a dark room, from a flash from say a camera?

After-image: The retina contains light receptors. A chemical reaction occurs inside these cells when stimulated by light, which in turn creates an electric signal that the brain interprets as light. Once the chemical reaction ends, the cells "re-charge" - slower with intense light. The stark difference between the bright light and dark room keeps the cells stimulated a little longer, creating an after-image.

Receptor recovery: The rods and cones, photoreceptor cells, respond chemically to light of various brightness; then they prepare for the next stimulus. When a very bright light such as a camera flash stimulates them, the response can overwhelm and it takes longer to recover. This leaves you with an afterimage, while the cells in the retina prepare again to respond.