Tumblr APIにて1投稿複数画像を実現するには

タイトル通り、TumblrというSNSでAPIが公開されていますが、
私が英語が読めないこともあり実現にだいぶ手こずっています。

以下にて新規投稿をできるかと思いますが、うまくこといかず...
なにかいとぐちをお教えいただけないでしょうか。
https://www.tumblr.com/docs/en/api/v2#posts---createreblog-a-post-neue-post-format

$content[0]['type'] = 'image';
$content[0]['media']['url'] = 'https://cdn.pixabay.com/photo/2016/03/28/12/35/cat-1285634__340.png';
$content[0]['media']['type'] = 'image/jpg';

$post_param = array(
'oauth_token' => get_access_token(),
'oauth_token_secret' => get_access_token_secret(),
'content' => $content,
);

prm($post_param);

$post_result = tumblr_oauth(get_dynamic_url(URL_POSTS), 'POST', $post_param);
$post_result = json_decode($post_result);
prm($post_result);

Array
(
[oauth_token] => bzxBB9xxxxxxxxxxxxxxxxxxLnXGIRr5
[oauth_token_secret] => 7xiyGDxxxxxxxxxxxxxxxxxxxxxxxxxxxl1xpe
[content] => Array
(
[0] => Array
(
[type] => image
[media] => Array
(
[url] => https://cdn.pixabay.com/photo/2016/03/28/12/35/cat-1285634__340.png
[type] => image/jpg
)

            )

    )

)
stdClass Object
(
[meta] => stdClass Object
(
[status] => 400
[msg] => Bad Request
)

[response] => Array
    (
    )

[errors] => Array
    (
        [0] => stdClass Object
            (
                [title] => Bad Request
                [code] => 0
                [detail] => Unknown error. Try again.
            )

        [1] => stdClass Object
            (
                [title] => Bad Request
                [code] => 0
                [detail] => Unknown error. Try again.
            )

    )

)