Apparently Blogger have made some changes to the ways images are used in headers.
Unfortunately, it did something very peculiar on the template I was using. Most of the picture vanished, and then when I tried to upload a new one for Christmas it just appeared as a narrow band in the middle of the default image. Most peculiar.
So I tried another template, and it was no better. It moved things around too much and looked far too wide on my screen - it would have been ridiculous on a widescreen monitor.
So I got rid of all images altogether and tried yet another template. I altered it until I liked the way it looked. Then I tried - just one more time - to import an image into the header. And it worked!
Thankfully, all the sidebar elements remained, so I didn't have to import those all over again.
Apologies to anyone who happened to visit my blog during the glitches of the past half hour. I hope it is now stable... I think I actually like this template better than the old one.