Lightbox doesn't show images loaded by entity_metadata_wrapper
In the lightbox configuration settings it allows you to define a certain class, so it popups whenever an image has that certain class. So all you have to do is to find a way to add a class to your image. 

Perhaps you could use the Image URL Formater module. Set your image field format to url. (So when it prints, it will print the image url instead of the actual picture)

then concatenate a string to add html to your code. 

print '&lt;img src="' . $image . '" class="lightbox"&gt;';