I haven't figured out what, or why, but the site somehow acquired code for setting the title (and description) to be an overlay on top of the actual content of the page. Which is bloody annoying. And I still don't know why, but eventually I've managed to get rid of the offending coding. Back to the layout I set up several years ago ("Simple Pale").
Do I still need to re-set the image width? Nope, seems to have gone away. Trying to work out where I previously broke things (the template-appended "email envelope icon" image was stretched across the page, instead of approximately square) is just ... frustrating. I don't know if it's in my code, or in it's interaction with Blogger. somewhere, their code surrounding the "email image icon" re-set the "image width" default that I'd set in a previous code block, so only that one image (in the rest of the template) got altered.
Some footling about because Blogger doesn't allow you to have "HEAD" or "BODY" tags in the post template, but doesn't say so until you try it.