How to store pictures in files on server to preserve privacy for some of those in WWW

[Bugs16]

I am planing to bulid www portal in PHP, where many pictures will be stored. I decide to store pictures in directories at the server(not DB) for performence reason. Some pictures will be accessible for all users from internet, and some (if user set them private) not according to session id. What is the best solution of this problem? Perforance is important. Shoud I use some mod_rewrite or move private files to other directory than public? John