Step 1: Press the "Print Screen" Button on your keyboard located to the left of the "Scroll Lock" Button
Step 2: If you have World of Warcraft installed normally go to
C:\Program Files\World of Warcraft\Screenshots
You should find images of the screenshots you took.
Step 3: Then to to a website such as ImageShack® - Image Hosting and upload the pictures there.
Step 4: Link the screenshot in this thread by clicking the "insert image" option when posting a thread, and type in the URL of the image
Once you have done this, your image should appear in the thread you posted in.