You can also click what I believe was the F2 or was it F3 key when in game to print the screen of what you are seeing in game.... The screenshot is then saved to a folder called screenshots or the like in the main minecraft folder. Granted I use a mac so I have to click the apple key + F2 (or F3, as I can't quite remember exactly which one it was)

Thanks

On your keyboard press the key that says "Print Screen" (the keys text will vary on keyboards)

Open up Paint or whatever image editing program you use, paste the image in (Ctrl+V) then save it (Ctrl+S), fo the best quality save it as a .PNG.

To upload the image to the Forums, make a new post and down the bottom you will have a box for adding a file, click "browse" then find where you saved your image. Now click "Add The File", you can then select "submit" and it will add it as an attachment