I’m pretty sure this is concept art done by Daniel Dociu (ANet’s art director).
He does use a lot of photos in his art, and by mixing/blending them together with all his painting & lighting techniques, he comes up with those giant environments.
If you want to imagine what GW2 could look like with a 3D engine 100x time more powerful that can render incredible 100% scale landscapes, you can check his work here : http://danieldociu.weebly.com/
Photos probably comes from his personal shots, or maybe from stock-photography that you can find on resources websites for artists.