Ah… the “hidden” part really threw me off. There is nothing hidden in the image. “Hidden” suggests steganography.
You interpret the image and (apparently) know that something is either conceptually behind something else or that before the transformation to the image there was some 3D nature to what is depicted (like a tree in front of a house in a photograph).
For example, if I take a screenshot of this page and add a big smiley in the center, you think of it as the smiley being “on top of” the screenshot. This is only in your mind; the picture does not contain such information and whatever was “below” the smiley is no longer in the image and cannot be recovered.
Of course, if the smiley would be transparent, some information is retained. And, as @feuerstein said, sometimes it is possible to change the image to make it appear like the smiley was never put there, but this requires guessing as to what was below it and remaking that from “scratch”.