i dont know, im on 3ds and there is only screen shots in certain games... idk about the wii u tough...

You can take screenshots to save them to your SD with the aroma plugin called ScreenshotWUPS

if you mean screenshots to post in here you have to go to the home menu while in a game then go to miiverse and you will get the option to post a screenshot of the moment you went to the home menu, you dont have to close the game