Generate thumbnail in php, posting to Azure Comput

I want to use Azure Computer Vision API to generate thumbnails for my Wordpress site. I'm trying to make it work in php with wp_remote_post, but i don't know how to parse the parameters ? It returns a thumbnail in really bad quality and default 500x500px. Any ideas on how to resolve this issue ?

function get_thumbnail($URL)   //* * * * Azure Computer Vision API - v1.0 * * * *
{
$posturl='https://api.projectoxford.ai/vision/v1.0/generateThumbnail'; 

$request = wp_remote_post($posturl, array(
 'headers' => array(
    'Content-Type' => 'application/json',
    'Ocp-Apim-Subscription-Key' => 'xxxxxxxxxxxxxxxxxxxxxxxxxxxx'),
 'body' => array('url' => $URL)
));

if ( is_wp_error( $request ) ) 
{
    $error_message = $request->get_error_message();
    return "Something went wrong: $error_message";
    } else 
    {
      return $request['body'];
    }    
}

EDIT 1

Thanks @Gary your right! Now the cropping is correct, but i got a huge problem with the quality! I'm using a trial but i see no info from Azure on downgrading the thumb quality for trial users. They are claiming to deliver high quality thumbnails, but if thats the standard it's totaly useless. I must have overlooked something i guess?

Of course Gary, if i get no correct answer on my quality question i will close the thread with your answer as correct.

According the description of Get Thumbnail, the width,height and smartCropping should be set as request parameters which should combined in URL.

However the second args in wp_remote_post() do not accept the URL parameters and will do nothing on them. So you need to combine the url first before set into wp_remote_post().

You can try to use add_query_arg() to combine your url first,

$posturl='https://api.projectoxford.ai/vision/v1.0/generateThumbnail';
$posturl=add_query_arg( array(
    'width' => 600,
    'height' => 400,
    'smartCropping' => true
), $posturl);