可能使用set_post_thumbnail()? (Codex ref.)

EDIT

要使用Post ID获取附件ID，请执行以下操作：

// Associative array of attachments, as $attachment_id => $attachment
$attachments = get_children( array(\'post_parent\' => $post->ID, \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', \'post_mime_type\' => \'image\', \'order\' => \'ASC\', \'orderby\' => \'menu_order ID\') );

$attachment = $attachments[0]; // ID of your single, attached image.

然后将其设置为特色图像：

set_post_thumbnail( $attachment );

我必须检查以确定；文档有点混乱。默认输出可能是对象。不管get_children() 会让你找到你的单独附件的ID，给出帖子ID。