How to Understand the Image of a Black Hole

We are about to see the first image of a black hole, the supermassive black hole Sagittarius A at the center of the Milky Way galaxy or maybe the supermassive black hole in the galaxy M87. But what is that image really showing us This is an awesome paper on the topic by J. P. Luminet: Image of a spherical black hole with thin accretion disk Astronomy and Astrophysics, vol. 75, no. 12, May 1979, p. 228235 Using my every day intuition I wondered: will we see the shadow of the bl