I'm definitely no expert, but I read about photographs for registration papers for Appaloosas. Obviously your not registering an Appy, but it might be the same concept.
They would need at least four pictures. One of each side of the body, and the front and back. The horse needs to be cleaned, and all for legs need to be visible. (Do not stack the horse, make them stand somewhat staggered.) Get as close to the horse as you can, and make sure they are standing on a flat surface like a driveway. Take pictures of extra details that might not be as notacable in the larger pictures; as in the hooves, eyes, underbelly, under the tail. If your horse has a mane or forlock that covers their face or neck markings then take one picture with it down, then put it up.
Hope that helps. ^^