After I Add a Photo, button should change to "Replace Photo"

Summary: In general, this is probably a symptom of the weakness of the Photo-editing UI. If I'm in the Instance Editor and have an empty Optional Photo, I click the "Add Photo" button to fill it. But after the upload is complete, it still says "Add Photo" -- indeed, there is no feedback that it has worked until I say Done.

This may become moot if we rethink the Photo-editing UI completely.