in a vacuum, you are safe putting liquid in via the high side port with the system off.
after it's running, in the low side port, you can still put in as a liquid, but VERY VERY slowly in small increments as a slug of liquid in the lines can damage the compressor.
most gauges have a sight glass (the center port on your gauges shows one in your pictures) dribble until you see liquid, then stop, and wait for it to flash out, then start again. never more than 10 psi above the operating pressure.