This is the long way, (I don't know the short one, if it exists)
Hit prtsc for the whole screen or alt-prtsc to capture the window that is in focus, now a snapshot is in your paste buffer.
Open up paint, ctrl-c to paste the screen into paint. Crop of the extra bits you don't want.
Save the 'file as', then add as attachment in the reply or created thread. Jpg is best (IMHO)
Who can tell us an easier way?