If you are no afraid of code , take a look at http://www.deluxeblogtips.com/p/taxonomy-meta-script-for-wordpress.html
it had a great class to add “meta fields” for taxonomies (including categories) and a very simple way to implement the built in media uploader to a custom field in the edit category screen.
Uploaded images’ ID are saved as a field with multiple values. Each uploaded image is treated as an attachment. This means it has all sizes that defined in Media section or in your theme functions.php file. So, you can use some built-in functions to get image URL, image size, etc.