Preview:
/**
 * Thumbnails option on Post Navigation Elementor widget
 */
add_filter('previous_post_link', 'sydney_child_post_nav_thumbnail', 20, 5 );
add_filter('next_post_link', 'sydney_child_post_nav_thumbnail', 20, 5 );
function sydney_child_post_nav_thumbnail($output, $format, $link, $post, $adjacent) {

	if( !$output ) {
 		return;
  }

  $divclass = '';
  switch ($adjacent) {
		case 'next':
			$divclass = 'custom-nav nav-next';
			break;
		case 'previous':
			$divclass = 'custom-nav nav-previous';
			break;
		default:
			break;
	}

  $arrow_prev = '';
  $arrow_next = '';
  if( 'next' == $adjacent ) {
    $arrow_next = '<span>&#10230;</span>';
  }
  if( 'previous' == $adjacent ) {
    $arrow_prev = '<span>&#10229;</span>';
  }

	$rel   = $adjacent;
  $thumb = get_the_post_thumbnail($post->ID, array( 100, 100));
  $title = '<div class="'.$divclass.'">' . $arrow_prev . $post->post_title . $arrow_next . '</div>';

	$class = '';
	if( !empty($thumb) ) {
		$class = 'post-nav-has-thumbnail';
	}

  $string = '<a href="' . get_permalink( $post->ID ) . '" rel="' . $rel . '" class="'.$class.'">' . $thumb;
  $inlink = str_replace( '%title', $title, $link );
  $inlink = $string . $inlink . '</a>';
  $output = str_replace( '%link', $inlink, $format );

  if( !$post->ID ) {
    return;
  }

  return $output;

}
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