The Image presented at a press conference today by the Event Horizon
Telescope collaboration is the result of collaboration between numerous
scientists around the world lead by EHT director Shep Doeleman.  They were able
to do this using the technique known as very long baseline interferometry
(VLBI).  

This technique creates an instrument with the highest possible angular
resolution ( smallest angle of separation between objects where we can distinguish
the features). Using this technique astronomers were able to see the details of
an object light years away. 

First they revealed the image created by observing the area
near the black hole at the center of  the galaxy M87.

Photo credit Event Horizon Telescope Collaboration.

To the eye of the general public the above may not be
impressive.   To be frank, it would not get a barely interested college
astronomy student to put down their phone.  

HOWEVER, what EHT described very well in their presentation
is the amount of work it took for numerous NUMEROUS scientist and engineers. 
This is not an image in the sense of what you get from a digital camera.  This
is a careful measurement of the radiation field, the light, in the vicinity of
a black hole.  The paths that they follow near the black hole appear to conform
to those predicted by Einstein’s General Theory of Relativity.  A theory which
was proposed almost exactly 100 years ago. 

It was asked why no image of Sagittarius A* Shep Doeleman answered this is
because of the dynamic nature of imagining it.  In simple terms.  The father away
something is the more it moves in your field of view.  In astronomy this can
mean it could be easier to take a steady photo of an object at a greater
distance.

Theoretical Implications.

Finding no large deviations from General Relativity kills
many more radical alternatives to General Relativity or constrains them quit a
bit.  Namely modified gravity models that seek to explain dark matter and dark
energy.  Looking at it, at first blush, the only modified gravity models that
may survive this test are those known as f of R gravity and its variants

Quantum theories of gravity (Loop Quantum Gravity and string
theory and/or gravitized quantum
field theory (Penrose et al) or relativized quantum field theory,  are
probably not tested by this observation.  Since at these size scales their
predictions would not deviate from General Relativity. The propagation of light
on these length scales from here are governed by gravity described by General
Relativity.   That said, future observations may be detailed enough to get an
idea about quantum gravity.

Consequently, this does not have implications for the black
hole firewall debate.  The firewall around the black hole would be something
you would be able to see if you fell into the hole IF it exist.  (I think it
does quantum physics all but requires it but this is a hotly debated matter.)

The youtube channel Veritasium has an interesting video on this.