Public domain images are free from copyright, either because they expired or never existed in the first place. However, public domain images and stock photography are not the same, and cannot always be used interchangeably. For instance, public domain images do not include all the security factors of a stock photo.
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).