Force image size for Facebook post feed

I'm using PHP Graph SDK to publish posts with images on Facebook, the source image size is 900x900px.

This is my code in summary:

require_once('php-graph-sdk-5.x/src/Facebook/autoload.php');$fbk_appid = 'xxxx';$fbk_appsecret = 'xxxx';$fbk_tokenpage = 'xxxx';$fbk_pageid = 'xxxx';       $picture = 'img1.jpg';$link = 'https://www.mysite.ext/page.html';$message = 'My message';            $name = 'Title under image'; $tags = '#tags...';$description = 'Description under title';                               $fb = new Facebook\Facebook(['app_id' => $fbk_appid,'app_secret' => $fbk_appsecret,'default_graph_version' => 'v3.3'    ]);try{    $response = $fb->post('/'.$fbk_pageid.'/feed',            array ('picture' => $picture,'link' => $link,'message' => $message,          'name' => $name,'description' => $description,                      ),            $fbk_tokenpage    );}catch(Facebook\Exceptions\FacebookResponseException $e){    // Error Graph ( $e->getMessage() )         }catch(Facebook\Exceptions\FacebookSDKException $e){    // Error SDK ( $e->getMessage() )           }// $response->getGraphNode(); //e.g. {"id":"1446491425007649_523376413379549"}

Upon publication the image is always resized to 1200x628px.How to force a custom image format (square for example)?

Thanks!