I get the photo onto the mac and the default program you look at photos with is called preview.
There a button for file>duplicate so I do that and the new file is called "...copy.jpg " then I can keep the original.
Then there is a Tools>adjust size button.
First, do any cropping you want to by selecting the part of interest and a crop option will appear on a menu.
I adjust the width to 1200 pixels or 800 px if its an upright (portrait) one.
Save as jpeg file type in "best" quality and the pic that came off your phone at 2MB will be about 400kb.
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