This image is from an instructional video and shows the link you should be able to use to replace the image with one of your choosing on your Facebook posts. My problems began with the (thing not said) idea that I could do this on a personal Facebook page – which I found impossible. I’ll leave it to you to ponder whether the problem was caused by my assumption, or by a lack of detailed explanation provided by the presenter. Either way, see below for the rest of the story.
After giving up on this idea, later I found out that you can change the image on your business page. OK, Facebook calls them; “Facebook Profile or Fan Page.” For the sake of clarity, your Facebook Profile is what I call a personal page, and your Fan Page is what I call a business page. So, here is an image from my business page.