It's not taking a photo. It's computation, based on a lot of real data, with a lot of inference and synthetic data. The idea is that it's broadly accurate but cannot capture fine detail.

The approach has been calibrated against known objects like NRAO 530, a quasar near Sgr A* that has some known properties, and it's been used on both Sgr A* and M87. And the team has admitted there's a lot of uncertainty about exactly how accurate it is.

Here [1] is a pretty good article about how it works.

There's overwhelming scientific consensus that Sgr A* and M87* are black holes. It's not really provable at this point, but I don't think anyone doubts it.

[1] https://www.discovermagazine.com/the-event-horizon-telescope...