The 4×6 aspect ratio comes from the film size as used in a still camera but not in a movie camera which is the origin of 35mm film ( for film the images were placed sideways on the film strip and for stills the images are placed lengthwise ). The film was inexpensive due to its use in cinema.
What Sells Best On Shutterstock?
Some types of photos that sell best are Portrait Photography, Food Photography, Landscapes (usually with people) and fine art photography (artsy).