[inebriate]

edit categories.tpl and change the stuff between the anchor (<a href="asdf">stuffbetweenanchor</a>) to <img src="images/{$categories[cat_num].category}.theimagefileextension" /> or <img src="images/{$subcategories[cat_num].category}.theimagefileextension" /> if you use subcategories

and place your images in the skin1/images directory as well as naming them exactly like their category names