The picture is of the central region of M87, taken at a wavelength where the gas is "optically thin".
The ring of bright light is pretty much exactly where it is expected to be for the synchrotron radiation emitted by the hot gas to have been gravitationally lensed by a black hole with the same mass as deduced previously from the motion of stars close to the centre of M87.
It is a "picture of a black hole", though all you see is the way it warps the light around it.
Your analogies are poor. It isn't like taking a picture of someone in a dark room. The room here is illuminated and the effects of the black hole on that light have apparently been seen quite clearly. That, by the way is the only method by which you can see someone in an illuminated room, unless you have eyes that detect the mid-infrared light they emit. Of course that option is not available for a black hole, since they emit no light at all.
Your other analogy is also curious. A cursory glance at the sky reveals it is full of air molecules. That is why it is blue. Your view though lacks the angular resolution to see individual air molecules, so you would not be able to see them.
Here, the resolution of the "telescope" is just good enough to reveal the essential structure and size of the photon ring.
Finally, why isn't it a picture of an accretion disk? As I said, it is expected that the disk is optically thin and actually much bigger than the imaged area. Emission from the disk would not be expected to show anything but a haze with a vague central concentration. Given the spatial resolution of the image, what has actually been seen is a sharp, bright ring.
The accretion disk around a black hole should be terminated at or slightly beyond the innermost stable circular orbit. This is at 3 times the Schwarzschild radius, but gravitational lensing would make it appear to have an inner radius of 3.7 times the Schwarzschild radius. The bright ring has a radius of 2.6 Schwarzschild radii; correct for a lensed photon sphere explanation, incorrect for the inner wall of the accretion disk (unless the black hole is 70% of the mass previously determined from stellar motions).
The orientation might be just about ok for an accretion disk explanation. If the rotation axis is aligned with the jet of M87, the disk should be about 17 degrees from face on. However, in order to see the inner parts having such a circular shape and well defined structure would require a geometrically thin and optically thick disk, the opposite of what is deduced from the physical conditions in the plasma.
To see more context, here is a wider view of M87 taken by Chandra, and from the Chandra team's blog post:
Chandra was used to observe M87 and other targets during the EHT campaign. While Chandra can’t see the shadow itself, its field of view is much larger than the EHT’s, so Chandra can view the full length of the jet of high-energy particles launched by the intense gravitational and magnetic fields around the black hole. This jet extends more than 1,000 light years from the center of the galaxy.
