"Insert > Screenshot" minimizes the Outlook window, lets you select a rectangular area to include in your screenshot (not necessarily the whole screen), and then automatically pastes that screenshot into your message.
Also, if you have OneNote running, at any time you can hit Windows+S to clip an area of your screen and copy to your clipboard. I use it all the time.
Also, if you have OneNote running, at any time you can hit Windows+S to clip an area of your screen and copy to your clipboard. I use it all the time.