Not training, but a LOT of time, patience, and practice (by photographer and handler). It may take an hour to get that perfect conformation shot. Usually one person on the shank, taking direction from photographer (left front back, right hind up, he’s leaning, etc). And another assistant in horse’s view to get “expression” and ears through a variety of tricks: play a recording of horse whinny is guaranteed to get ears and alert eyes. Crinkle peppermint wrappers, rattle a plastic bottle of rocks, hold a plastic bag low and shake it gently, etc…anything to direct horse’s attention and interest, all while keeping the feet still in position.
A few horses just walk out of the barn, look off into the distance and pose while the camera shutter clicks…but that’s not training, just a classy horse who loves to show off!