What do you have for View/Message Body As? It must be 'Original HTML' to show remote embedded images.

"View/Message Body As" IS set to "Original HTML." The problem persists. Note that I did say that some images are displayed correctly, but some are not - the display is a rectangular box outline but with no image inside the box. This problem only appeared after the latest Windows 10 Pro update. I do have a system backup taken before the update, but I'm concerned that if I restore from that backup, Microsoft will again force an update, and the problem will resurface.

I doubt the problem is due to the Windows update, but other possible factors could be an updated security program that is scanning emails, or the images are being blocked by your mail service before the messages are downloaded to TB. If that's the case, it may help to access the account through webmail and whitelist (by adding to webmail contacts) or unblock images from that sender. Also, check under Tools/Options/Privacy that senders have the correct Allow/Block setting.

The reason some images display and some don't could be because the former attach the images while the latter use remote links to images.