I also have difficulty believing it's a real photograph. I just stick 'I want to believe' at this moment.
On the other hand speaking from experience with postprocessing DSLR photographs I don't see any signs it isn't a photograph.
It does look like a highly processed photo though, like several layers of sharpening, de-grain, adjustment of colors/contrast/..etc.
Only thing which makes me really doubt is the fact there is no reflection of the astronaut in the eye.
There's supposed to come more information about this case.