List posts with post thumbnail

Hi, im trying to display list of posts that have postthumbnails.
The code im using is below:

<ul>
			<?php query_posts( 'cat=43,-5&posts_per_page=4' ); ?>
			<?php while (have_posts()) : the_post(); ?>
				<li id="<?php the_ID(); ?>" class="pic">
					<a href="<?php the_permalink(); ?>">
               			<?php echo the_post_thumbnail('thumbnail'); ?>
           			</a>
				</li>
			<?php endwhile; ?>
		</ul>

Can someone help me to edit it so that the loops gets the post that have postthumbnails and ignores the ones that dont.

Thank you