Thank you for the plugin!
Installing and activating it i am getting the warning:
Missing argument 2 for wpdb::prepare(), called in [......]plugins/category-icons/category_icons.php on line 1047
Missing argument 2 for wpdb::prepare(), called in [...]/plugins/category-icons/category_icons.php on line 1338
I might be wrong but i believe that since you do not pass any variables into those queries you do not need to use prepare.
So for me, removing prepare from the two queries also removed the warnings.