Get the featured image in WordPress when set, if not a fallback will be loaded.

wp-featured-image.php

<?php
  if( has_post_thumbnail() ):
    $featured_thumb_id = get_post_thumbnail_id();
    $featured_thumb_url_array = wp_get_attachment_image_src( $featured_thumb_id, 'large', true );
    $featured_image = $featured_thumb_url_array[0];
  else:
    $featured_image = get_template_directory_uri() . '/assets/images/default.jpg';
  endif;
?>