April 8, 2012
This was a tougher find than you'd think. There are several plugins to assign images to specific WordPress categories, but after trying out several that have strange workflows (or just plain don't work at all), I settled on Maga Category Images.
It would be nice if the image files were uploaded on the Edit Category page itself, but in practice it's not a big deal to head over to
Settings->Category Images to choose from a dropdown of categories and upload an icon or thumbnail.