The heavy infantry model is used with the horse as the heavy cavarly model. The guy on the horse in the picture is the light cavalry model and it shows up fine in game. Heavy Cavalry model just usually takes preference over even if there is more light cavalry in the army. Its not the first time they have done this when they show the picture of the units. If I remember correctly they did the same with Christian Holy Order unit pack the heavy infantry on a horse is the heavy cavalry model and they only showed the light cavalry model on the horse in that one too.
Still the picture is indeed showing the wrong horse with the light cavalry one as its using the heavy cavalry horse, but the rider is the light cavalry one. I think there was some artistic licence taken to show all different models without showing something twice.
