A photograph, is a piece of art work. Just as much a piece of art work as a painting, a pencil sketch, a pen and ink drawing, etc. The principles of composition, highlight, shadow, shape, form, etc are the same. The difference is in the tools used to get the image and the time it takes to do so. Also! As with any other piece of art work, a photograph should either tell a story, or evoke some kind of feeling or emotion (hopefully positive or at least not overly negative) in the viewer. If it does that it's a success.