http://dev.wp-plugins.org/wiki/CategoryIcons :
Copy the category_icons.php file into your wp_content/plugins directory. Activate the plugin from Plugins admin menu.
Once the plugin is activated it creates a "Cat Icons" panel under "Options". This will allow you to manage category icons.
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Examples
This example will insert the icon associated with the current category:
<?php get_cat_icon(); ?>
This example inserts icon for the category with ID '5'. The image is fitted withing rectangle 100x100. If the image is smaller than desired, it is expanded.:
<?php get_cat_icon('cat=5&fit_width=100&height=100&expand=true'); ?>