I just noticed that the photographs overlapped with the sidebar. Why I spent as much time on changing the template as I did last night without noticing the problem--that's something I can't explain, other than that I just forgot the problem from the last time I tried to change the template and the photos happened to be low enough on the front page not to be next to any sidebar content and the lack of a sidebar dividing line left the side space clear.
Sour grapes version: the type on Minima was too hard to read and was hurting my eyes, the white background made the dark ad banner much more obvious, and that template had a problem with line spacing that was undermining its prettiness. Now, back to substance!
UPDATE: Blogger has some sort of bug that is causing the template to jump back to Minima when I edit an old post. What could cause that? Let's see if it happens this time ....
FURTHER UPDATE: Ah, good. I have no idea why the problem cured itself and I don't trust it not to happen again. It's definitely a good idea to keep a copy of your template in a separate document, so you can copy it back into the window if it decides to revert to an earlier incarnation.