tumblr - How to post text with picture by tumblr api?

Question

Im trying to post the text with picture as follows

$params['type'] = 'text';
$params['markdown'] = true;
$params['state'] = 'published';
$params['title'] = $title;
$params['body'] = '![this is image](http://xxxx.jpg)';

new HTTP_OAuth_Consumer($consumer_key, $consumer_secret_key)->sendRequest('http://api.tumblr.com/v2/blog/xxx/post', $params);

but text displayed as plain text without a convert by markdown. If I once clicked preview button, text is displayed correctly.