这就是我使用的函数，也许有更好的方法。

$attachments = get_children(
                            array(
                                  \'numberposts\' => -1,
                                  \'order\'=> \'ASC\',
                                  \'post_mime_type\' => \'image\',
                                  \'post_parent\' => get_the_ID(),
                                  \'post_type\' => \'attachment\'
                                  ));

               $first_attachment = reset($attachments);
               //$last_attachment  = end($attachments); or last image
               echo wp_get_attachment_image($first_attachment->ID, \'full\');