The approach relies on identifying the “Featured Image”, which wordpress interprets by the ‘single-post-thumbnail’. See Featured Images Support for more info.
<li class="shareFacebook"> <a rel="nofollow" href="http://www.facebook.com/share.php? s=100 &p[title]=<?php the_title();?> &p[url]=<?php the_permalink(); ?> <?php if (has_post_thumbnail( get_the_ID() )) : ?> <?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), 'single-post-thumbnail' ); ?> &p[images]=<?php echo $image; ?> <?php endif; ?> &p[summary]= <?php echo urlencode(substr(strip_tags(the_excerpt()), 0, 250)); ?>" title="Share this post on Facebook" target="_blank">Facebook</a> </li>
Sadly these aspects of sharer.php are very poorly documented by facebook, the best documentation we have found on this is at daddydesign.com. Hope this helps.