Once your photo is uploaded to 500px and shared on your Profile it is open to be viewed by the public. You can add photos to Sets, which by default are publicly visible, however they have the option of Private and Password protected. Portfolios are also public, by default, but have the option of Private.
Publicly visible photos will appear on the Fresh page upon upload, if they have a title, at least 3 tags and are in a category. They may also appear on Upcoming, Popular or Editors Choice, based on several variables, such as it's Pulse and age.
You can manage your photos using 500px.com/organizer. See how here.
Need more help? Email firstname.lastname@example.org or tweet @500px_help