Found the answer on neogaf since the gamefaqs community appears to be too elite to answer faqs anymore. (talking about the forums here)

"You can't "take a screenshot," per say, in that you don't first capture and save it anywhere. What you do is, when you want to share a screen, hit Home when the game is showing what you want, go to make a Miiverse post, and the image attach button will be prepopulated with whatever your game was showing when you hit Home."