Show Post Excerpt in Image Attachement

First refer to this answer by @MikeSchinkel. You can put the function in functions.php

Next, just add echo robins_get_the_excerpt($post->post_parent); below the image attachment. Like this :

<div class="caption">
    <?php echo robins_get_the_excerpt($post->post_parent) ?>
</div>

Please note that this code is untested.