Using your built in Chromebook tools to capture your screen or portion of a screen.
Then any captured images can be found in your Files app as a download for further use or editing. (See the shortcuts below)
Another extensions to consider for screenshots and editing:
Activity: Using your screenshot skills, take a screenshot of this page and add some edits like cropping, or even some markups.
Google Apps 101 by Kurt Wismer is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.