Every time a public visitor (i.e. a person who is not logged into your account) views an image, the number of unique views get logged and displayed in the Stats tab of each project.
Views are unique to each image, so if you move one image to a new project - those stats will be carried over. You can also remove all projects stats by hitting the 'Reset' button at the bottom of the Stats tab.
For more detailed statistics, you can integrate with an existing Google Analytics account via your Project Settings. For help setting up your Google Analytics integration, please read this guide.