My memory card frequently shows old thumbnail images yet when I open
them they are the new ones that have replaced the old ones. I tried
refreshing the Windows Explorer folder to no avail. If I modify the
picture in any way the thumbnail shows the most recent version OK.
What can I do to get the thumbnails to always display the most
recent picture short of modifying them?

I suspect you're viewing the images directly on the memory card,
rather than copying them to a folder on your hard drive.

Yes. Using a Dazzle USB card reader.

This means
that Windows Explorer is writing a hidden file (called Thumbs.db, I
believe) for the thumbnails directly onto your memory card.

Yes.

You're also, presumably, using the setting on your camera that
re-starts image numbering every time you change cards, rather than
continuing
from where you left off.

Yes.

This means that your camera is writing a
file with the same name as one of your earlier files, but with new
contents. But it doesn't know anything about the thumbnail database,
so it just leaves the old thumbnail there.

If you just delete the Thumbs.db file it will be re-created
correctly from the most recent images.

Sounds logical. I'll try it. Thanks.