The two default locations come from the Pictures folders in the current user account and the Public account
When you click on that link, you'll see the Pictures Library Locations dialog box. As you can see, the two default locations come from the Pictures folders in my user account and the Public account. You'll use the Library Locations dialog box to add and remove locations, as well as keep tabs on which locations are in the Library. (Each of the other three Libraries has a similar Library Locations dialog box.)
When you click the Add button, you'll see a standard browse dialog box called Include Folder in Pictures where you can add folders from any location to which you have access.
Greg Shultz is a freelance Technical Writer. Previously, he has worked as Documentation Specialist in the software industry, a Technical Support Specialist in educational industry, and a Technical Journalist in the computer publishing industry.