suddenly, just a bit ago, many of the book cover images for books in my transaction archive disappeared!!  Anyone else?

Mine are still there.

Sounds as if your browser may have purged a cache of downloaded images.
Did you recently run any utility that "cleans up unused files to improve performance"? (Some virus checkers offer this service).
Do the images come back if you do a "hard refresh" of the browser window?  (hold down the shift key and press the reload (circular arrow) button in the browser.
If that doesn't restore the images, will you provide a sample ISBN this happened to? 
I can see if the image is missing for me as well.

Images are back without me doing anything........