you can take a screenshot by pressing L+PAD DOWN+SELECT all combined and you will be in luma menu, go to Take Screenshot and it will take it, then push your SD card to your computer, go SD/luma/screenshots and copy the screenshots to your desktop, go to your wii u and copy the

screenshots to a carpet and download WiiUShell in the homebrew appstore, go to the carpet where are your screenshots and press A to check the photos, zoom with L joystick up and go to miiverse to post the image

First you need to get Screenshot from the HB App Store then follow what Pancho said