Abstract: Relativistic jets throughout the universe transport energy from small scales near a galaxy’s central supermassive black hole to extragalactic distances.  These jets may be powered by the spin energy of the black hole via the Blandford-Znajek (BZ) mechanism; however, BZ energy extraction has not yet been confirmed observationally.  In this talk, I will discuss what polarized images of synchrotron radiation from close to the event horizon can tell us about black hole magnetic fields, jet launching, and spin. Near-horizon Event Horizon Telescope (EHT) images in linear and circular polarization strongly suggest that the accretion disk in M87* is magnetically arrested.  I will show that the pattern of linear polarization in EHT images directly probes the direction of electromagnetic energy flux, and that the EHT images indicate that electromagnetic energy is flowing outward on horizon scales around M87*.  The spiral pattern of polarization vectors in EHT images is directly connected to the underlying magnetic field structure, potentially allowing for precise measurements of black hole spin. Future observations of M87 will be sensitive enough to detect faint emission both closer to the event horizon and farther downstream in the jet launching region, potentially enabling a definitive test of the Blandford-Znajek mechanism for powering extragalactic jets.