Once you're playing the game, hit the square button and select 'Stickers & Decorations". When you do that you need to scroll to the 4th tab which says, "My Pictures".
At the very top, under the section "Functions" select "Take A Photo".
Once you have taken a photo hover over it and press square to bring up more options. When you do "upload" will be one of the options. Click on it and the pin/trophy is yours!
Here's a video showing you what to do as well. I don't take any credit for this video.