Add custom CSS class to product images

If it’s still relevant, you can add a class to the product thumbnail the following way.

function loop_product_custom_thumbnail() {
  echo the_post_thumbnail('thumbnail', array('class' => 'your-class-name'));
}
remove_action( 'woocommerce_before_shop_loop_item_title', 'woocommerce_template_loop_product_thumbnail', 10);
add_action( 'woocommerce_before_shop_loop_item_title', 'loop_product_custom_thumbnail', 10 );