This code works alone, but it doesn't work implemented in WordPress

I have this code that uses gd libraries, and it can add text on a image. If i run this code alone or on a static page, it works. But when i run it in WordPress, it appears a broken image.

This is the original code.

<?php
      //Set the Content Type
      header('Content-type: image/jpeg');

      // Create Image From Existing File
      $jpg_image = imagecreatefromjpeg('sunset.jpg');

      // Allocate A Color For The Text
      $white = imagecolorallocate($jpg_image, 255, 255, 255);

      // Set Path to Font File
      $font_path = 'font.TTF';

      // Set Text to Be Printed On Image
      $text = "This is a sunset!";

      // Print Text On Image
      imagettftext($jpg_image, 25, 0, 75, 300, $white, $font_path, $text);

      // Send Image to Browser
      imagejpeg($jpg_image);

      // Clear Memory
      imagedestroy($jpg_image);
    ?>

And this is the same code implemented on WordPress, which I have problems.

<?php
/*
Template Name: Tarjetas
*/
?>
<?php
  //Set the Content Type
  header('Content-type: image/jpeg');
?>
<?php get_header(); ?>
    <div class="containing">
    <?php
		$user = wp_get_current_user();
		$id = $user->id
	?>
    <?php
  // Create Image From Existing File
  $jpg_image = imagecreatefromjpeg('https://www.alvarols.com/califica/wp-content/themes/mexicotecalifica/images/tarjeta.jpg');

  // Allocate A Color For The Text
  $white = imagecolorallocate($jpg_image, 255, 255, 255);

  // Set Path to Font File
  $font_path = 'https://www.alvarols.com/califica/wp-content/themes/mexicotecalifica/font.TTF';

  // Set Text to Be Printed On Image
  $text = $id;

  // Print Text On Image
  imagettftext($jpg_image, 25, 0, 75, 300, $white, $font_path, $text);

  // Send Image to Browser
  imagejpeg($jpg_image);

  // Clear Memory
  imagedestroy($jpg_image);
?>

    </div>
 <?php get_footer(); ?>

I hope you can help me.

Thanks.