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.
If you’re running the latest version of Windows you can use the Snipping Tool. Click on the start button and then Snipping Tool. Follow the directions to take a new “snip.”