Arc has a built-in screenshot feature called Capture.

Take a Capture by clicking the Site Control Center then selecting the Capture a portion of the page icon, or by hitting Command-T and typing "Capture" and then clicking and dragging a rectangle to capture a portion of your current webpage.

Once you've taken a Capture you can add it to a new or existing Easel if you like. More on that here.

If you'd like to take a standard macOS screenshot in Arc, you can use the macOS default of Command Shift 4.