WC_Product::get_image PHP Method

    public function get_image($size = 'shop_thumbnail', $attr = array(), $placeholder = true)
    {
        if (has_post_thumbnail($this->get_id())) {
            $image = get_the_post_thumbnail($this->get_id(), $size, $attr);
        } elseif (($parent_id = wp_get_post_parent_id($this->get_id())) && has_post_thumbnail($parent_id)) {
            $image = get_the_post_thumbnail($parent_id, $size, $attr);
        } elseif ($placeholder) {
            $image = wc_placeholder_img($size);
        } else {
            $image = '';
        }
        return str_replace(array('https://', 'http://'), '//', $image);
    }

/**
 * Filter the document table row cells to add product thumbnail column data
 *
 * @param string $table_row_cells The table row cells.
 * @param string $type WC_PIP_Document type
 * @param string $item_id Item id
 * @param array $item Item data
 * @param \WC_Product $product Product object
 * @return array The filtered table row cells.
 */
function sv_wc_pip_document_table_row_cells_product_thumbnail($table_row_cells, $document_type, $item_id, $item, $product)
{
    // get the product's or variation's thumbnail 'shop_thumbnail' size; we will use CSS to set the width
    $thumbnail_content = array('product_thumbnail' => $product->get_image());
    // add product thumnail column as the first column
    return array_merge($thumbnail_content, $table_row_cells);
}