I sometimes take a screenshot on a Mac (OS X Mavericks), by CMD-Shift 4, and resize it to 50% width and height in Preview.
But when posted on the Internet (for example, on an online forum or on ibb.co or imgur.com) , it is double size again. I tried 50% and then resolution changed from 144 to 72 for the original image, and the same: it is still double size. I had to do a 25% and also 72, in order that it is the proper size.
It is really weird. Why 25% and not 50%, and how does the 144 or 72 matter? (or does it?)