add thumbnail to serach results

I have tried to add a call for the thumbnail to the content.php in theif ( is_search() ) section but the thumbnail still isn't showing in the search results.

		if ( is_search() ) : // Only display Excerpts for Search ?>
            <div class="entry-summary">
                <?php the_post_thumbnail('thumbnail'); ?>
                <?php the_excerpt(); ?>
            </div><!-- .entry-summary -->

I also tried to remove this section but that doesn't work either. How do I get the thumbnail to show in the search results?