Accretion disks are a complicated system which produces a wide variety of emission across the electromagnetic spectrum on a variety of timescales. The diagram below shows (some) of what is going on schematically, along with typical sizes scales (bars with arrows with sizes in parsecs (pc)). Details about how the "central engine" of supermassive black holes and AGN works and exactly how the energy is generated is an ongoing topic of research, which the Event Horizon Telescope images will very likely contribute to.
Friction and turbulence from the material orbiting the black hole causes the accretion disk to heat up and emit light. The inner parts of the accretion disk get the hottest and tend to emit at the shortest wavelengths (X-rays normally) with longer wavelength (optical, IR, radio) normally coming from the material further out and above the disk which is photo-ionized by the X-rays. Depending on the density of the material in and around the accretion disk, the angle we are viewing it from and whether the accretion disk flares up at larger radii, will control what the accretion disk/AGN will appear as which part of the spectrum will be seen most strongly.
In the case of the sub-mm images produced by EHT, the emission is though to be coming from the ionized plasma closest to the inner edge of the accretion disk (the red region around the black hole in the bottom zoomed-in plot) via synchrotron emission as electrons are accelerated by the magnetic fields of the plasma. This is in contrast to the jet radio emission which is on a larger scale and separation and often extends several galaxy radii which can be seen in e.g. this VLA/VLBI composite radio image.
There is some additional information on accretion disks and how they work at this EU Strong Gravity research site and a more technical review article in this conference proceedings.