If you only want to capture a portion of the screen, press Shift + Ctrl + Change Windows key. Using your cursor, outline the area you want to take a screenshot of by clicking and dragging. This will create a custom window.
When you take a screenshot, it will appear briefly in a little window at the bottom of the screen. You can click the little window to open the image. If you need to access all of your screenshots, use the Files application from the main Chromebook menu.