On my site, the photo gallery is only showing the yearbook photo (and very large). I don't believe that was the way it was when I first started setting up my site, so I must have changed some setting somewhere, but don't remember where! Is there a way to edit the gallery on the profile page?

Sure...
Go to Manage Classmates, Edit/Enter Classmates, pick one, and click on Edit Profile. Then click on the Photos thumbnail at the upper right to see what (if anything) is in their photo gallery.
At that point, you can click on any photo to either display it or hide it (turn on/off the green check mark), and add or delete photos, etc.

How can there be photos in their 'gallery' when they have not joined? Are you adding them?

What's New shows they added a photo, yet they have not joined. Very strange.