  Following the 2019 release by the Event Horizon Telescope Collaboration of
the first pictures of a supermassive black hole, there has been an explosion of
interest in black hole images, their theoretical interpretation, and their
potential use in tests of general relativity. The literature on the subject has
now become so vast that an introductory guide for newcomers would appear
welcome. Here, we aim to provide an accessible entry point to this growing
field, with a particular focus on the black hole "photon ring": the bright,
narrow ring of light that dominates images of a black hole and belongs to the
black hole itself, rather than to its surrounding plasma. Far from an
exhaustive review, this beginner's guide offers a pedagogical review of the key
basic concepts and a brief summary of some results at the research frontier.
