When everything is just right and the hunter is looking perfectly stylish, pressing the A button will take the picture. Pictures taken in the photo mode are saved in an in-game album as well as the Switch's album on the HOME menu. Any picture from the in-game album can be displayed as a decoration in the hunter's room.
When you exit the game, Steam should pop up a window with the screenshots you took for that session so you can delete or upload them as you see fit. Also you can access your screenshots in the upper left hand corner of your Steam page. Click view tab, scroll down to screenshots.