Controlling which picture is used for FaceBook thumbnail

The following code inserted into the header.php passes off the thumbnail in the article. <?php if (is_home()) { echo ‘<meta property="og:image" content="http://www.amarketplaceofideas.com/wp-content/uploads/2013/04/Wavy-Arm-Guy.jpg"/>’; } elseif (function_exists("has_post_thumbnail") && has_post_thumbnail() && !is_home() ) { post->ID), full ); ?> </p> <meta content="<?php echo $thumbnail[0]; ?>" property="og:image" /> <?php } ?> <p>