There are times when sharing a post to Facebook or other social media websites gets the wrong image, even if you changed it. This happens because they have a cache set up, and in order to make it re-build the cache you’ll need to make them re-create it.
Go to https://developers.facebook.com/tools/debug/, enter your URL and and press debug. You’ll see there the info that Facebook takes from your website. If the image is wrong, press the button Fetch new scrape information, it will make Facebook re-build their cache.