I think you have to download it manually or you can get a screenshot app from the homebrewed app store on your wii u. hope this helps :3

There's a screenshot plugin on the HB Store for Aroma, but to post them on Miiverse you need to press the home button, go to post a message, and press the picture icon on the right. You can choose wether the top screen or bottom screen, then download it on a smart device after.