...and now, there's no eye in my blog header!
It seems that was the problem with the way it was displaying, as many of you noted. Or at least, some changes Blogger made behind the scenes that violently disagreed with the way the standard template I'm using handled the image file. I've tried resizing it, but it isn't having any of it, so the simplest thing is just to take it out for now.
With thanks to @Shugism who diagnosed the problem via Twitter.