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?)

  • 1
    Are you sure you are changing the actual size of the image or maybe you are just changing the view (zoom) on the image in the current opened application to view that image? – pallox Aug 29 '18 at 13:01
  • Do you change the number of pixels in the image or just alter the PPI meta data information? – Thorbjørn Ravn Andersen Aug 29 '18 at 13:06


I'm not quite sure what you mean. I tested it with a screenshot (see above) and reduced the resolution to half, uploaded it here, it's still the new resolution that I chose in Preview.

Maybe you can try the following:

  • Change the resolution in Preview
  • Close the image
  • Right click on it, check Get Info, then "More Info", there you should find the current resolution which should be slower than before
  • If this does not work for you, maybe try a tool like ImageMagick that offers a lot of options for manipulating images.


  • Hi Harpe, remember to provide a solution in an answer :) – pallox Aug 29 '18 at 13:04
  • You're right, I added some steps to test or try something different. – Harpe Aug 29 '18 at 13:59

Why not change it by size (inches or pixels) instead of percent. Then you could be sure of the size.

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