To draw on a screenshot, follow these steps:
Click the Camera button.
Drag your mouse to crop a part of the screenshot that you want to send.
You can also take a screenshot of the whole screen:
...or upload an image from your computer:
...or paste an image from the clipboard:
...or acquire an image from scanner or a webcam:
To draw, you can either use shapes:
Or draw with a pencil or highlighter. Just click the "Pen Tool" menu and select the tool, color and width. Then click outside the menu window to confirm your selection and then begin to draw.