Add WooCommerce product category image full width under the header of the Storefront theme.

storefront-header-category-image.php

/**
 * Add WooCommerce product category image full width under the header of the Storefront theme.
 */

function woocommerce_category_image() {
    if ( is_product_category() ){
	    global $wp_query;
	    $cat = $wp_query->get_queried_object();
	    $thumbnail_id = get_term_meta( $cat->term_id, 'thumbnail_id', true );
	    $image = wp_get_attachment_url( $thumbnail_id );
	    if ( $image ) {
		    echo '<img src="' . $image . '" alt="' . $cat->name . '" />';
		}
	}
}
add_action( 'storefront_before_content', 'woocommerce_category_image', 2 );