There are many companies online that will sell your images but none of them them quiet as large and easy to work with as Shutterstock.
With Shutterstock you retain ownership of your photos and can sell them directly on your own website or you can list them with a different service (as long as that service does not require exclusivity).
In this video we demonstrate how to create a Shutterstock account, upload pictures (and videos!), detail the categories and key words and submit them. We also examine why some images are declined.