You can tell it's a photograph.
Look on the left side on the edge of the mountain, you can see places where the wind has swept up some of the snow, creating a kind of swirl on the silhouette of the mountain. That would only be in a photograph or a photoshopped render.
The sun looks realistic enough to me. Just because the disk diameter is small and its coming through lots of cloud cover doesn't mean it's CGI.
Looks like it could almost be CGI, but we have a ways to go until we can create truly photo-realistic images such as this. Terragen images always appear very clean; there's normally not much grain (although this can be added in post), while photographs usually have some amount of grain, especially in low light conditions like this image. Computer generated images are always too clean, too perfect looking.