převod image na square image format

if(isset($_POST["submit"]))

  { 
 //load an image from the local drive folder.
   $filename =  $_FILES["pobrazek"]["name"];
   $img = imagecreatefromjpeg($filename );

  $ini_x_size = getimagesize($filename)[0];
   $ini_y_size = getimagesize($filename )[1];

  //the minimum of xlength and ylength to crop.
   $crop_measure = min($ini_x_size, $ini_y_size);
   // Set the content-type header
   //header('Content-Type: image/png');
   $crop_array = array('x' =>0 , 'y' => 0, 'width' => $crop_measure, 'height'=>
   $crop_measure);
   $thumb_img = imagecrop($img, $crop_array);
   imagejpeg($thumb_img, "slides_prodej/", 100);
}
?>

Fatal error: Uncaught Error: Call to undefined function imagecreatefromjpeg() in C:\xampp\htdocs\upload_image_photo.php:379 Stack trace: #0 {main} thrown in C:\xampp\htdocs\upload_image_photo.php on line 379