By default, each of your photo albums is visible to any other member. You can change that on an album-by-album basis. To do so, go to Account.
For desktop users, click My Classmates® in the top portion of any page. (For mobile users, click , you may have to expand to make your selection) then select Privacy and scroll down to Share info on my profile. You'll have the option to select who can view each of your albums: Public (everyone) or Only friends & communities.
Note: These settings only apply to photo albums posted on your profile. Your Now and Then profile photos, as well as other profile content such as your story, profile notes, etc. are visible to all other members.