Probably the best way to get a good idea of what kind of stock images sell is simply to look for images that have been used around you. Images in magazines and brochures, images in displays and advertisements, images in text books and on websites. Many of those images will have been purchased from stock libraries.
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).