Appending a class attribute to woocommerce single product image

 - $image_title         = esc_attr( get_the_title( get_post_thumbnail_id() ) );
            $image_link         = wp_get_attachment_url( get_post_thumbnail_id() );
            $image              = get_the_post_thumbnail( $post->ID, apply_filters( 'single_product_large_thumbnail_size', 'shop_single' ), array(
                'title' => $image_title
                ) );
            $attachment_count   = count( $product->get_gallery_attachment_ids() );

            if ( $attachment_count > 0 ) {
                $gallery = '[product-gallery]';
            } else {
                $gallery = '';
            }

            echo apply_filters( 'woocommerce_single_product_image_html', sprintf( '<a href="%s" itemprop="image" class="woocommerce-main-image zoom" title="%s"  rel="prettyPhoto' . $gallery . '">%s</a>', $image_link, $image_title, $image ), $post->ID );

Above is the code from product-image.php file of woocommerce. I want to add a class to the image of single product. I am unable to find out the way for it. Please help.

Thanks