You say he's posting them as his own, but mouseover the thumbnails and the original artist's name appears, njeneb and porcupinefloyd to name two. Maybe he's posting stuff he likes; whether that is permissable is another matter, but the guy isn't signing his name to others's work in the examples I saw at the first link. The second link thumbs seem all to be signed by the same artist, and I don't recognize the renders so I don't know whether they're his or not.