1. Open the Screenshot in Preview
2. Select “Adjust Size…” from the Tools menu
3. Change the Width and Height to 50 percent of the original
4. Change the Resolution to 72 pixels/inch

I wish to do this on CLI with a generic command (percent, not actual width and height).

[GUI instructions with Preview](http://www.macissues.com/2014/03/29/adjust-retina-macbook-screenshots-to-be-actual-size/)