Idk how to post screenshots but you can find your screenshots in your luma folder on your SD Card if you plug it into your computer but idk if you can post the screenshots onto MiiVerse because screenshots was never a feature on the stock 3DS (i think)

Misinfo, Screenshots are a stock feature of both Miiverse versions. Just have the game suspended. Go to the Miiverse icon and go to its respective community and tap post. you should see a small picture icon right there...