Show image captions on custom post type.

Hi I am working (for free) on a charity web site collecting war stories. Each story is a custom post type. To add images to the story I have the following code. Unfortunately the images are not showing captions in lightbox. Do you know what php I should add to the code to get the captions to show?

				$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 class="lightbox" 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>';

If you need access login below.
Many thanks
Rob