Just pause the game while playing (by pressing the Home button), launch Miiverse, and make a post. When composing the post there should be a + button on the right. Press this and you can attach either the picture being displayed on the TV or the picture displayed on the Gamepad.
This post copyright ©2014 by Popple. All rights reserved. No part of this post may be reproduced in any form without the written permission of the copyright owner. This post is a work of fiction. Any resemblance to actual events was done on purpose.