I don't believe that photo was enhanced at all. I've known this horse and both his parents, since he was born and many of his photos are much alike. That is, except for his different silver colours throughout the seasons. Certainly the light in the arena, might have had a slight effect upon all horses. You can see him in this video, when in a darker coated season. AUSTIN/ Gypsy Vanner Stallion - YouTube
You can see his white in this video, also sometimes looks as though it is glowing.