I use the print screen button. Then open up a photo editing software (I use Corel Paint) and hit paste. That will copy it in. You can then add text/ resize etc. I frequently have to also downgrade the resolution, I try to get it under 100k, but the first one I did was 250k as I didn't have time to muck about.
The forum software uploads it as an attachment so people can see the size and download if they wish, this is good as it keep sthe thread small.
Edited cause I can think faster than I type... which isn't hard... I can still talk faster than I can think though... not a good set of traits to have!
Cheers
Rob