So you're wondering: How can [person] put a photo so clearly? This blog post has the answer (If you have a Macbook computer)
What you need: A copy of Tomodachi Life, a 3DS or 2DS, and an SD Card. Make sure the SD Card is inserted.
Step One: Take a photo in Tomodachi Life with X (for the top screen) or Y (for the bottom screen.)
Step Two: Take out the SD Card. Insert it in one of the holes on the side of the computer where it would fit.
Step Three: iPhoto will pop up a few seconds later. Go to the folder of the SD Card, and it will have your Tomodachi Life screenshots.
Step Four: Import either everything or the photo you want to upload. It will appear in "Last Import."
Step Five: Minimize this page so you can see your desktop. Drag your photo in "Last Import" to the desktop.
Step Six: Your photo is now on your desktop. Upload it here, and there you go!