How do I get the thumbnails to only show when there is one?

[KyleA]

In the home.php file there is a section of text that says:

				<?php if( get_post_meta($post->ID, "thumbnail", true) ): ?>
				 [img]<?php echo get_post_meta($post->ID, [/img]" alt="<?php the_title(); ?>" />
				<?php else: ?>
				 [img]<?php bloginfo('template_url'); ?>/images/thumbnail.png[/img]" />
				<?php endif; ?>

How do I modify this if I only want the thumbnails to display when one is actually attached to the post? In other words, if I don't have a picture associated with the post, how do I get the content to show up on my blogs homepage without the default thumbnail.png next to it?

Thank you,
Kyle A. Christopherson