How to resize a screenshot image on a Mac so that it is "regular size" when posted on the Internet?
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.


http://cactuslab.com/imagemagick/