“Maybe if you do nothing it will just go away” (said by no one responsible).
A few months ago I noticed images in some of these posts were not rendering correctly – but only in some circumstances. If I pulled up pages with individual posts the images appeared correctly. But when I looked at the main page of the blog (multiple posts in descending order, by date) some images did not.
Today I was poking around thinking I might take another stab at troubleshooting and noticed the problem is gone.
Another win for procrastination!