functions.php

<?php
/**
 * Add classes to subcategory images.
 *
 * @param $category
 */
function kanuka_category_custom_thumbnail( $category ) {
	$output = '';
	ob_start();
	woocommerce_subcategory_thumbnail( $category );
	$output .= ob_get_clean();
	$output = kanuka_add_class_to_tags( $output, array( 'img' ), 'object-contain max-height-100%' );
	echo $output;
}
remove_action( 'woocommerce_before_subcategory_title', 'woocommerce_subcategory_thumbnail', 10 );
add_action( 'woocommerce_before_subcategory_title', 'kanuka_category_custom_thumbnail', 10 );