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.
Note: On a Chromebook to save the image to the clipboard, click “save to clipboard” on the pop-up screen in the bottom left hand corner before you navigate away from that page.