For a complete screen: press Ctrl key and the Multi-screen key (located on the top row of the Chromebook keyboard.)
For a part of the screen: press Ctl key, Shift key, and the Multi-screen key. Your cursor will become a crosshair, so you can click and drag across the part of the screen you want to grab.
After you let go of the keys, the system will copy it to your clipboard, so you can paste it into the document, slide, web site, etc.
We used this when creating avatars on Avatarmaker.com. The images wouldn´t download, so the students just did a screen grab of their avatar, and pasted it in their book talk slide stack.