WooCommerce snippet to customise Single Product image thumbs.

wc_product_image_thumbs.php

<?php /**
 * Single Product Thumbnails
 *
 * @param array $args
 * @param int $attachment_id
 * @param string $image_size
 * @param bool $main_image
 * @return array
 */
function bones_gallery_image_html_attachment_image_params ( $args, $attachment_id, $image_size, $main_image )
{
    $args['class'] = esc_attr( $main_image ? 'wp-post-image' : 'wp-post-image-thumb' );
    return $args;
}