Looks like this was a Facebook problem. Even though I set it up correctly, Facebook was still showing some kind of error. Here’s what I did that worked:
I loaded the Facebook object debugger at this URL: https://developers.facebook.com/tools/debug
and gave it the URL of my blog posting.
It showed that the og:image tag was correct but it claimed that there was an error so it was serving up a the default image.
I clicked on “Fetch new scrape information” which caused it to refresh itself.
Then it worked correctly.