In addition to the keyboard shortcuts below, you can use Awesome Screenshot Extension for Chrome- This is exactly what it says-awesome. It allows you to annotate images and add boxes. Sometimes when I choose “selected area” it crops it funny, so I choose “visible part of the page” and do the cropping myself. You can then save the image to your computer or copy it to use directly in another application.
For a partial screenshot saved as a file- use command+shift+4
For a full screenshot saved as a file – command+shift+3
For a partial screenshot copied to the clipboard (the only one of these I’ve memorized!)- command+shift+option+4
For a full screenshot copied to the clipboard – command+shift+option+3