When some one is unhappy with their photographs it may be a problem with composition. There is a set of rules that artists have developed over the centuries. The one rule of composition most often quoted is the Rule of Thirds, but there are others that are very useful.
Another concept that is helpful is to reduce an image to its essentials. Anyone can take a picture of a horse but it is important to represent the essential horse. Such a photograph should convey the idea of what it means to BE a horse. What a horse feels, how he reacts to his surroundings, etc. I use a horse as an example but I think that everyone will grasp the concept.