The weapon first of all. A like new M4, it DOES have a magazine however its hidden behind the sling but who over there has access to a brand new M4 with an Eotec site and front handgrip? Kind of an unusual weapon to see a Jihadi with.
What did it for me and make me think it was a fake was his hand on the pistol grip. Look at the white outline.. almost as if it was cut and pasted without touching up that area.
Also the shadows look off, the depth of the picture looks off as well. I could be wrong but my vote is the shooter was added to the image after the fact.
That would also be consistent with the angle of the natural light (from behind at ~2:00).
So, is this guy an idiot? Shooting at IDF soldiers with no magazine in the weapon? Or, is he so quick that he can move his hand up to release the mag and back to the trigger guard before the smoke from the end of the muzzle dissipates and the last spent round casing gets more than 2 feet from the weapon? If so, he's good maybe his name in al-Barney-Omar and they only give him one bullet a day.
I'm not concerned so much about the white line on the wrist area, in that its highlighting and consistent with illumination throughout the rest of the image. The black region immediately adjacent to that in no way that can be reasonably explained as being shooter's shoulder or hair of somebody in background.