Some instructions:
1) Screenshots are taken ingame using the PrtScr key. It's up there beside Scroll Lock which you probably have bound to Hearthstone. Don't hit it by accident.
2) You can find your screenshots in C:\Program Files\World of Warcraft\Screenshots.
3) You will need a program like
http://www.irfanview.com/ in order to browse and convert your images to .jpg format. In the next patch they intend to fix this I understand where your screenshots will automatically be in .jpg format.
4) Go to the Screenshot Gallery section of the webpage, and choose the album you would like to upload to. WoW for pictures of WoW. The other for any sort of pictures.
5) Once in the correct album you will see a button like this:
. Click this. Fill everything in. Ignore the size (height and width) requirements, as long as the pic is less than 2megs it will upload.
6) Wait for approval.
7) Pic have a comments section. Comments are hilarious!