How toHow to display only icons in the sidebar

You have 2 solutions :

1. With the widget, just put
icons_only=true
in the parameters :Icons Only

2. Manually, i.e. in the sidebar.php page of your WordPress theme, you must use the put_cat_icons function:

put_cat_icons(wp_list_categories('title_li=&echo=0'),'icons_only=true');