If you use a PC press Prt Scr ((upper right on your keyboard) Print Screen), then open Photoshop (or Outlook, Word etc etc) and make New file. Now press Ctrl+V (Paste) and then screenshot will appear. Save as Jpg and then upload on the forum.
On a Mac press the apple key, shift and 3 at the same time and the image will be on your desktop. Load this into the forum. (this is why i would never get a MAC, too bloody complicated)