I downloaded the background image separately. It's fairly obvious to me that the background image does not appear to be from a single photo. My impression is the sand, the rocks and the sea are originally a separate photo from the sky and distant islands. The sharp edges of the rocks against the sky and sea and unclean difference between the sea and the islands tends to suggest this both in color and shading. This severe sharpness may have been deliberate for special effect (and if it were just a single photo.) But the concept of combining photos together for special effect can be very powerful.