Another option if you don't have a Mac. Download the free app "Paint.net". If you have other imaging software on you computer: when the picture is on your desktop, right-click to bring up the context menu and select "open with" and click on paint.net. The fourth item in from the left is "image". Select and scroll down to resize.

Several options are available such as: lock height & width; quality; pixels or inches; etc.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store