inthegrass, once the time you get is up press the 'Print Screen' button on your keyboard (to the right of the backspace, if not it should be close by), open up MS Paint and paste in the screen shot, adjust the pic so it's not of the full screen and save it. You can either just attach the pic to your post or upload it to somewhere on the net and put the image URL in your post.
Well that's how i do it