Take a picture of your screen in Preview on Mac
You can use Preview to take a picture of all or part of your screen.
Note: You can also take pictures of the screen using the Screenshot feature in the Finder. See Take screenshots or screen recordings.
In the Preview app
on your Mac, open the PDF or image you wan to capture in a picture.Do any of the following:
Entire screen: Choose File > Take Screenshot > From Entire Screen.
Part of the screen: Choose File > Take Screenshot > From Selection. Move the pointer to a corner of the area you want to capture, drag across the area, then release the mouse button.
Single window: Choose File > Take Screenshot > From Window. Click the window you want to capture.
The picture opens in Preview, named “Untitled.” You can annotate it by clicking the Show Markup Toolbar button 
and using the tools in the Markup toolbar.