Always seems to work

I also got it to work with featured images with a link:

<?php 
if ( has_post_thumbnail()) {
  $custom_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'full');
	echo '<a href="' . $custom_image_url[0] . '" title="' . the_title_attribute('echo=0') . '" data-rel="lightbox" >'; the_post_thumbnail('folio-featured-image' );
	echo '</a>';
    	}
    	else {
			echo '<img src="' . get_bloginfo( 'template_directory' ) . '/images/default-image.jpg" />';
	     }
?>