Attach image from API Topics

[Askancy]

I'm using the post Topics api, but I can't figure out how to attach an image to the discussion...

function CreateTopic($forum,$title,$post,$author) {
	$url = 'https://forum.****.it/';
	$apiKey = '*******';
	$point = '/forums/topics';
	$endpoint = $url. 'api'. $point. '?key='. $apiKey;

	$options = [
				'form_params' => [
				'forum' => $forum,
				'title' => $title,
				'post'	=> $post,
				'author' => $author
				]
	];
	$client = new Client();
	$request =  $client->post($endpoint, $options);
	return $request;
}

I can't find anything in the documentation to attach an image to the discussion...