I'm pretty sure this is a photo-manipulation. You are right, in that the image of Altair about to stealth-assassinate a Templar is ubisoft art, and is located on the box art for the game's PS3 release.
However, in the original image, the background is low contrast characters in a fading white background. In this case, it's much more of a scene.
I still don't believe it should be on DeviantArt, because it hasn't been changed enough to truly make it original content.