Thread:Merrystar/@comment-5889551-20180224015044/@comment-11001-20180225025310

It's a setting in Special:WikiFeatures. The "Add a photo to this gallery" button is missing because the gallery can only be edited in source mode. Perhaps Help:New Image Galleries would be useful - it has more information about how the new galleries work.

As you are not an admin, you will either need to ask them to turn it off or just edit the whole page in source mode.