You can take a screenshot with the standard OS X shortcuts (Command-Shift-3, Command-Shift-4, etc.) then paste into a text cell. Quiver lets you save images inside text cells, including web images (which will be saved locally so they work offline).
If you like a quick workflow, use Command-Control-Shift-3 to take a screenshot and save it to clipboard, then paste in Quiver.
If you like a quick workflow, use Command-Control-Shift-3 to take a screenshot and save it to clipboard, then paste in Quiver.