CBS News has an article about the current state of image alteration citing a number of famous photo manipulations that have made the news in recent years.
It includes this quote:
“‘The analogy I always like to draw is, imagine a pile of sand […] And when does it go from a couple of grains of sand to a pile? And surely, taking one grain of sand on and off doesn’t fundamentally change the pile of sand. But at some point, it’s no longer a mound of sand, and it’s just a couple grains. But where did that transition happen?'”