Conditional statement in image.php

I should welcome a bit of help with an edit to my image.php file…

I am using the inbuilt WordPress Gallery and when a visitor clicks a thumbnail in the blog it opens the corresponding large image.

My aim is to have the image description in a grey box (using class wp-caption from the CSS) below some of the images. At the moment, I acheive this using the following bit of code within image.php:

<?php echo wp_get_attachment_image( $post->ID, 'large' ); ?>
<div class="wp-caption"><?php if ( !empty($post->post_content) ) the_content(); ?></div>

However with this code, if I don’t wish to enter a description for a particular image, an empty thin grey box will show under the full size image.

How should I alter the code so as to prevent the box from appearing if the image hasn’t been given a description?

I can provide a link to the blog if required. Thanks in advance.