Thanks for that. I have an addendum. This works well for bare images for iphone. It does not work if a bordered div contains an image and its caption — it will float left. I looked it up and there is a bug:
"The Kindle app on iOS (iPhone and iPad) has a long-standing, notorious CSS bug: it frequently ignores margin: 0 auto; on div containers if those containers house images. While the app respects text-align: center; directly on an <img> tag, it strips the auto-margins from a parent block, snapping the whole div container to the left."
There are kludgy work-arounds offered, neither working at for me. One of them requires wrapping the image and caption in a P tag, but that does not work either.