Lazy load with featured image.

I am using the following to output my featured images with the class .lazy

<?php the_post_thumbnail('post-thumbnail', array( 'class' => lazy)); ?>

I need them to be outputted the following way:

<img class="lazy" src="img/grey.gif" data-original="img/example.jpg" width="640" heigh="480">

Any help would be much appreciated.

Thanks