How can I add a photo to a gallery?
Go to your gallery page, click "Add media". If the photo that you want is not already in the libary, click upload, drag the filename from your PC to the Drupal window, and click "upload". The pic gets automatically added to the library.
If the pic is already in the library, find it, select it (click it) and hit Submit.
For either method, you can do multiple files at a time.