A list of oddities:
1) There are shadows (yes, shadows) underneath the BoBs and Cyborgs. This is clearly not in the game.
2) The Fighter corpse on the left is actually an M1 sprite.
3) There are no Cyborgs on Waterloo Waterpark.
4) The Fighter on the right has a rather strange appearance (two eyes, human-like fingers, ultra-bendy staff). Reminds me of the image at the top of the FAPTA page on The Pfhor
I would venture to guess the photo is a composite screenshot, much like this one, which GrimCleaver kindly pointed out way back in June 2012 (see the bottom of the Official Marathon Screenshots FAPTA page).
Vale,
Perseus