I need a system for file delivery

I am making a website for my wife's photography business. The server is hosted remotely with PHP/SQL, etc... but no access to the server OS.

I would like to implement a system where I can have users login and obtain their pictures whenever they want. The users really don't have to have a login and password, just some mechanism where they can pull their files, and the files are relatively secure.

It would take a very long time to code this from scratch, so I am looking for a system that is already prebuilt. The website itself is almost finished.

I don't mind paying a bit for such a system, but free / open source is what I am looking for.