WordPress: Conditional Thumbnail or Image

<?php
  /* LISTING PAGE EXAMPLE */
  if (has_post_thumbnail()) {
    echo get_the_post_thumbnail(get_post()->ID, 'thumbnail');
  }
  else {
    echo '<img src="/assets/img/deltawire-thumbnail-default.jpg" alt="" />';
  }
?>

<?php
  /* FULL POST PAGE EXAMPLE */
  if (has_post_thumbnail()) {
    echo get_the_post_thumbnail(get_post()->ID, 'medium');
  }
?>