So far I've not published a post in which it happened; it's happening when I'm writing or editing a post, and makes it very difficult to tweak the images once they're uploaded.. I'm wondering if one of the images I downloaded was dodgy in some way - but it doesn't seem to be affecting my other blogs at all.
Having said that, I did look at the code for an old post in the 'edit post' screen, and one of the images there started zooming in a way it hadn't before, and now appears on the blog larger than it originally was - the first image here: http://woolwinding.wordpress.com/2012/06/13/is-it-a-bird-is-it-a-plane/ (yes, I know it's insane)...
Have tried browser tweaks. It's the same situation on Safari and Chrome, not tried IE.