  In recent years, unprecedented progress has been achieved regarding black
holes' observation through the electromagnetic channel. The images of the
supermassive black holes M87$^{*}$ and Sgr A$^{*}$ released by the Event
Horizon Telescope (EHT) Collaboration provided direct visual evidence for their
existence, which has stimulated further studies on various aspects of the
compact celestial objects. Moreover, the information stored in these images
provides a new way to understand the pertinent physical processes that occurred
near the black holes, to test alternative theories of gravity, and to furnish
insight into fundamental physics. In this review, we briefly summarize the
recent developments on the topic. In particular, we elaborate on the features
and formation mechanism of black hole shadows, the properties of black hole
images illuminated by the surrounding thin accretion disk, and the
corresponding polarization patterns. The potential applications of the relevant
studies are also addressed.
