in index.php find: WHERE i.image_active = 1 AND c.cat_id = i.cat_id AND i.cat_id NOT IN (".get_auth_cat_sql("auth_viewcat", "NOTIN").")
Replace with: WHERE i.image_active = 1 AND c.cat_id = i.cat_id AND i.cat_id NOT IN (".get_auth_cat_sql("auth_viewcat", "NOTIN").") AND i.cat_id IN (XX,XX,XX,XX)
Replace XX with category IDs you wish, you can add as many categories as you wish.