Lightbox from url

Thanks Yigit.
I have a custom post type set up
e.g. http://goldhillmuseum.org.uk/story/sidney-william-alner/
Uploaded images display as thumbnails. This is the code from my loop-index.php.

				$content_output .= '<div class="story-images">';
				$content_output .=  '<span></br><strong>Images: </br></strong></span>';
					if( $images ): 
					$content_output .=  '<ul>';
						foreach( $images as $image ):
						$content_output .= '<li>';
						$content_output .= '<a href="'.  $image['url'].'">';
						$content_output .= '<img src="'. $image['sizes']['thumbnail'].'" alt="'.  $image['alt'].'" />';
						$content_output .= '</a>';
						$content_output .= '</li>';
						endforeach;
					$content_output .= '</ul>';
					endif; 
				$content_output .= '</div>';

I was hoping that within the image link I could add a class or something that opens the images in lightbox and not full size as it does now.
Cheers
Rob

Thanks Andy login below.

Hi Ismael,
I haven’t added another lightbox plugin.
It is adding the”lightbox-added” class but it doesn’t seem to be working. This is the generated html for one thumbnail:
<a style="position: relative; overflow: hidden;" class="lightbox-added" href="http://goldhillmuseum.org.uk/wp-content/uploads/2016/01/Shaftesbury-War-Memorial-1.jpg"><img src="http://goldhillmuseum.org.uk/wp-content/uploads/2016/01/Shaftesbury-War-Memorial-1-80x80.jpg" alt=""><span style="display: none;" class="image-overlay overlay-type-image"><span class="image-overlay-inside"></span></span></a>
The only difference I can see is that it is within a custom post type called Stories. The rest of the theme functionality seems to be okay. This is the loop-index.php:

$content_output .= '<div class="story-images">';
$content_output .=  '<span></br><strong>Images: </br></strong></span>';
	if( $images ): 
	$content_output .=  '<ul>';
		foreach( $images as $image ):
		$content_output .= '<li>';
		$content_output .= '<a href="'.  $image['url'].'">';
		$content_output .= '<img src="'. $image['sizes']['thumbnail'].'" alt="'.  $image['alt'].'" />';
		$content_output .= '</a>';
		$content_output .= '</li>';
		endforeach;
	$content_output .= '</ul>';
	endif; 
$content_output .= '</div>';

Thanks
Rob

Hi Vinnie,
No it is not resolved. Lightbox is not working on a custom post type within the Enfold theme. The first code is the generated html for one thumbnail. The next code is the code in loop-index that calls the images.
Sorry if I am repeating myself but I don’t know how to answer your question unless I summarize what I have already written.
Thanks
Rob

Thanks Ismael that fixed it. I tried lightbox-added but not just lightbox.
Nice one
Cheers
Rob