I made a Client Photos category blog list page for listing all my clients galleries. I then made an article for each, added that clients gallery then logged onto the frontend of my site and password protected each of the client articles by clicking the padlock that appears. When you logout those articles can only be viewed by entering their specific password.
This pluginis easy to install (you also need the Core Design Scriptegrator plugin installed) and then you just login through the frontend and click on the padlock to password protect that article. Brilliant.