The big challenge is that black holes are quite small. In order to say that you have "seen a black hole", it is necessary to rule out that you have seen somethi

The big challenge is that black holes are quite small. In order to say that you have "seen a black hole", it is necessary to rule out that you have seen something similar to a black hole but larger than what general-relativity would predict. The black hole at the center of M87 is large (about the size of our solar-system), but it is 55 million light-years away. If I computed things correctly, it subtends about 70 picoradians on the sky. That is about the same angular size as Alan Shepard's lunar golf ball as seen from Earth. So yes, if a black hole is at the center of a galaxy, its (non-)image will be in any image of a that galaxy. But, proving that you've seen a black hole means ruling out that it is some other kind of compact object that might be somewhat larger than a black hole. The only imaging-based way to do so is to image its event horizon, which requires resolving the angular size of the black hole itself.