I'm still in PC-helpdesk-mode! So to fill the down time here's how I screen grab:
There's a button on most keyboards in the top row (often to the left of the Delete button) marked Prt Sc. When you want to keep a copy of what's on your screen, including during video playback, click that button and the action is captured and saved to your clipboard.
Open some simple image manipulation software. On my Windows based system I use Microsoft Paint which can be found in the Accesories folder on the Start Menu.
Choose 'paste' from your software menu and the saved screen image will appear. 'Save as' a jpeg, gif, png, or whatever you fancy, to a folder of your choice.
Tweak, annotate and distribute to your heart's content!