start playing the game you want a screenshot from, then when you want to take a screenshot, press the home button on the gamepad/wii remote, and instead of going to thr home menu, just click on miiverse, then go to where you want to post, and when you are typing your message,

and to the right of the message before you send it should be a tab called "screenshots" click on it, and choose your screenshot, (it might not appear when you are typing,) and then send.