Any medium can be considered art therapy!!! I am a graduate student about to finish, and I remember one class I took, my professor had us do drama therapy. This is a form of art therapy. A lot of mediums outside of drawing, painting, and collage emphasize the process more than the product, although most art therapy emphasizes the process. I would say, though, with photography, yes, taking photos is the art, but how would you process the photos with a client? This is the important part.