it came with last year's update - the feature is accessible from the title screen when you turn on the game, like this:
(x)
in it, you can have your mayor use an emotion and have a specific pose - to upload your picture, you can use miiverse the same way you did your reference
you can also do a pose in game and take a simple screenshot but the amiibo camera allows your picture to be bigger, so it's easier for me to use