Thumb resize stuck?

So, I’ve had a problem with ‘Create a Thumbnail.’ It resizes all images to be 400 pixels wide, even when I choose a thumbnail size smaller than that, and even when the original images are smaller than that.
Just to be sure, I turned off my plugins and did a fresh install of 1.2, and it is still doing that.
I vaguely recall using the thumbnail without problem in the past, but it’s been awhile, and I don’t know what could have changed.