/templates/single-product/product-thumbnails.php

<?php
/**
 * Single Product Thumbnails
 *
 * @author 		WooThemes
 * @package 	WooCommerce/Templates
 * @version     1.6.4
 */

if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly

global $post, $product, $woocommerce;

$attachment_ids = $product->get_gallery_attachment_ids();

if ( $attachment_ids ) {
	?>
	<div class="thumbnails"><?php

		$loop = 0;
		$columns = apply_filters( 'woocommerce_product_thumbnails_columns', 3 );

		foreach ( $attachment_ids as $id ) {

			$classes = array( 'zoom' );

			if ( $loop == 0 || $loop % $columns == 0 )
				$classes[] = 'first';

			if ( ( $loop + 1 ) % $columns == 0 )
				$classes[] = 'last';

			$attachment_url = wp_get_attachment_url( $id );

			if ( ! $attachment_url )
				continue;

			printf( '<a href="%s" title="%s" rel="prettyPhoto[product-gallery]" class="%s">%s</a>', esc_attr( $attachment_url ), esc_attr( get_the_title( $id ) ), implode( ' ', $classes ), wp_get_attachment_image( $id, apply_filters( 'single_product_small_thumbnail_size', 'shop_thumbnail' ) ) );

			$loop++;
		}

	?></div>
	<?php
}