Not discouraging more information here, but FYI, there have been a few threads on this that you may find interesting:
When Did People Start Smiling in Photographs? featuring /u/Axon350
When did smiling in photographs become common? - more from /u/Axon350
When showing teeth (in art, photograph and portraits) became culturally acceptable? And why it wasn't before? - /u/GeeJo gives a quick survey of smiling in art
These threads are all archived, so if you have follow-up questions for any of the users, just ask here and tag their username