Once your photo is uploaded to 500px and shared on your Profile, it is open to be viewed by the public by default. You can add photos to Galleries, which are also publicly visible by default, however they have the option to be made Private and only make visible to anyone using the shareable URL. Portfolios are also public, by default, but have the option to be made Private.
Publicly visible photos will appear on the Fresh page upon upload. Only the first 3 photos per batch will go into Fresh. They may also appear on Upcoming, Popular or Editors Choice, based on several variables, such as the Pulse and age of the photos.