The image that has been captured by the Event Horizon Telescope is actually an image of the accretion disk (the glowing region) that surround the black hole (the dark shadow).

The black hole is imaged as dark because light cannot escape from the event horizon of the black hole. Actually, the black hole is at the center of the dark disk and we cannot get any information about what is happening inside the event horizon. On the other hand, the accretion disk is the source of all physical processes associated with black holes like jet formation, outbursts etc. This region is bright and glowing because this region is filled with plasma at very high temperature and usually intense magnetic fields.

The image that has been captured by the Event Horizon Telescope is actually an image of the photon ring (the glowing region) that surround the dark shadow.

The black hole is located at the center of the dark disk and the disk radius is about 2.5 times that of the event horizon. The photon ring results from light rays that orbit around the black hole in the near-field region before escaping to infinity, where they arrive near a ring-shaped critical curve on the image plane. The shadow represents the interior of the critical curve.

The "photon ring" is a region of enhanced brightness near the critical curve that arises if optically thin matter emits from the region where unstable bound photon orbits exist. The light rays that comprise the photon ring can orbit many times through the emission region and thereby pick up extra brightness. Since the optical path lengths become arbitrarily long near the critical curve, the brightness can become arbitrarily large (neglecting absorption).