Now the template uses “shop_catalog”(woocommerce_thumbnail) images for “slider for gallery thumbnails” and for the “8theme – Products Widget”. How can I use “shop_thumbnail”(woocommerce_gallery_thumbnail) there?
In woocommerce 3.3.3 version, the plugin authors removed image settings at all even from customizer. But we keep the previous version of their settings http://prntscr.com/ik093w, so image size you can change via Customizer > woocommerce > product images https://xstore.helpscoutdocs.com/article/89-product-images (do not forget to regenerate thumbnails using Regenerate thumbnails plugin to apply changes). But 8theme product widgets use WP thumbnails size (Settings > Media) to avoid too big image in that area.
Feel free to ask if you have any other questions.