What is the light source in the black hole images? What is the light source in the black hole images?

I've searched the site before asking, as well as tried to google it. But perhaps the answer is buried due to the recent news.

My understanding, is that the ring in the images is an illuminated accretion disc.

What I could not find explained is the light source, and by extension, why is a bright accretion disc – which is round by definition – a proof for light bending? (I'm not disputing the fact.)
The radiation being detected in this case is mostly synchroton radiation, caused by energetic electrons spiralling around magnetic field line, rather than thermal radiation, but it comes from the same place (the disk). The actual evidence for light bending is, I believe the dark area in the middle, which is several times larger than the actual event horizon and is caused because the light that would have been coming to us from that direction has been bent away.