I don't believe that it erodes the public trust any more than it already has. For those who oppose the administration, this is just one lie added to the top of the pile. They don't have any trust to erode.
For his supporters, it's yet more proof that they are in the right, and that opponents are weak cowards. The fact that the image was manipulated doesn't really register. At most, it's just a way of making the truth clearer -- the real truth, even if the photographer didn't happen to click the camera at just the right moment to capture it the conventional way. For them, their trust keeps going up.
This is the standstill we'll be at for quite some time. To erode public trust further, there would have to be consequences that people tied back to the manipulated pictures. I don't see that happening today.