Thanks for the reply. I see that it indeed did what I want, but I don't understand why! The alignright certainly moved the image to the right in the post. But, it is still left in the thumbnail-excerpt, which is what I want. But, I guess I don't understand the interplay between the display in the post which seems to follow the html in the post...and the rendering in the excerpt on the home page.
I'm willing to live with the uncertainty
unless you can point me to the "why" of that change.
Apart from that, can I modify a template to enforce this? I rummaged around in functions.php, but didn't see where this could be enforced. I fear that if I did so, I'd just mess it up when I brought in the next image and placed it with the image tool?