A “hand” is 4 inches. a 15 hand horse is 5” tall at withers. Shorter than the average female.. at the withers.
a 17-18 hand horse is rare.
The traditional standard for height of a horse or a pony at maturity is 14.2 hands (58 inches, 147 cm). An animal 14.2 h or over is usually considered to be a horse and one less than 14.2 h a pony, but there are many exceptions to the traditional standard. In Australia, ponies are considered to be those under 14 hands (56 inches, 142 cm), For competition in the Western division of the United States Equestrian Federation, the cutoff is 14.1 hands (57 inches, 145 cm) The International Federation for Equestrian Sports, the world governing body for horse sport, uses metric measurements and defines a pony as being any horse measuring less than 148 centimetres (58.27 in) at the withers without shoes, which is just over 14.2 h, and 149 centimetres (58.66 in), or just over 14.2½ h, with shoes.
Looking good so far. I like the poses. Very natural.
Is the background someplace you have convenient access to? If so, you might want to take two photos. One would be for the background, and the other would be the same angle with a real human stand-in that you could use as a reference for lighting. The thing notice about it the most is that the background photo is taken on a cloudy day so the light is diffuse, but there is still a visible direction from which the light is coming from, which is visible on the trees in the background. To me it would appear to be the above right. The main light on the model is coming from the left. The easiest fix would be to reverse the photo. I’m not sure if it would be possible in D/S or whatever you use, but most image editors should allow it.
The other possibility would be to match the primary light to the photo and lighten any shadows with bounce lights. You have a lot of green, so maybe a bounce light pointing up with a green hue. you could also use a bounce light to simulate bounced light in the atmosphere from the left, right, and top, pointing towards the model from the direction of the camera. Try a light blue-gray. You might even be able to leave the shadows off or have them a low intensity to avoid shadow artifacts where you don’t want them. If D/S has a distant light try those. If not, then try spot lights. Open the angle way up and set the half angle high as well for spots. Keep the intensity low.
I won’t dilute your thread with my own images, but I’ve started a thread where I posted a picture of CG dinosaurs in a photo of my back pasture, and one where I tried to make it look as if my nephews were feeding their new pet. They’re not perfect, but if you wish, look closely at the areas in shadows and the underbellies to see what I did with lights. http://www.daz3d.com/forums/viewthread/5995/
The important thing is to take what works for you, and leave the rest. I’m no huge expert in lighting, there are others here that blow my mind when I see what they do. I wish they still had the various galleries working so you could see some primo examples from Bryce, D/S and Carrara.