The image has been touched up for the postcard. Look closely and you will see that it doesn't follow the contours of the hillside at all. The lines of the horse's legs ignore the valleys and folds of the terrain. I would say that after the photograph was taken ( which would have been black and white of course), it was then hand coloured in awful green ink and that obscured the feint trace of the horse. The finishing touch was to paint the horse on to the photograph and the photographer (or his assistant) guessed its appearance.