Taking screenshots with the Wii U is easy. It’s a shame there’s not much of a point to it at the moment, but you can share your in-game moments with other Wii U owners in Miiverse.
When you’re in-game and have a moment you want to share, click the Home button on your GamePad. This will bring up the Home Menu. From the Home Menu, click Miiverse. This will launch the Miiverse hub for the game you are playing. From here, click post. If you look in the top right-hand side, you will see a picture icon. Press this and both the TV and GamePad screens will be available for selection.
You can only select one photo per post, so you can’t select both the top and bottom screens at once, which is a shame. Press the one you wish to share, type in any text you want and press the post button to publish your screenshot to Miiverse.
At the moment, you can’t save the screenshots to your SD card and import them to any place more significant that Miiverse, so unless you want to share your leaderboard scores or need help with a specific area of a game, I don’t think many users will actually use this feature.