You can require your customers to create an account and log in to view galleries in the section you have your client photos in.
First edit the section you want the customers to be required to be logged in to view. Next scroll down and down the right side click Require to view and then the require visitors to be logged in to view pages in this section.
Now if the customer is not logged in, when they go to that section or a page / gallery in that section, they are prompt with a log in screen.
