Screenshot: In a game, app of home screen, open Mii Plaza and start a post. Touch the button at the top right of the editing menu, then select the screen you want to use. This is blocked in some games/apps.

Photo: Might be different on a 3DS, but on Wii U, you have to make a picture using your phone. Open it in cafémp (on HB store, might need an addon to allow the screenshot), then upload it like above.

im on 3ds and it just says check back soon =/