It is my understanding that you can mitigate this to an extent by making larger scopes or employing techniques like interferometry. I haven't found an absolute 'hard limit', only a limit based on the size of the resolving device.
I'm generally a moron in this area though so I'm certainly missing large swaths of information.
On the other hand, earth is a finite sized object, and so are photons, so there are only so many photons to work with no matter how large your detector.
I'm too lazy to try the calculations now, but if the nearest black hole is 2800 ly away, I guess the size of lens to resolve a dinosaur have to be much bigger than the solar system. Probably a few light-year [calculation needed]. Does the lens reach Proxima Centaury? Does the lens collapse and form another black hole?
Also, the black holes are big, but they are more far away than big, so the angular size of the area that does some interesting lens effect is very small. Then even if you have a giant lens and collect all the light that the black hole has blended (more than 90° or something), you will get a extremely dim image. But photons are discrete, and visible photons have a minimal energy, so the image will be composed of a very small amount of dots, and not something smooth where you can see anything.
I'm generally a moron in this area though so I'm certainly missing large swaths of information.