This sets a pages og:image property to the 'featured image' of that page. Facebook uses og:image when displaying images for posts to Facebook.

functions.php

<?php
    // Adding the Open Graph in the Language Attributes
    function add_opengraph_doctype( $output ) {
		return $output . ' 
          xmlns:og="http://opengraphprotocol.org/schema/"
          xmlns:fb="http://www.facebook.com/2008/fbml"';
	}

    add_filter('language_attributes', 'add_opengraph_doctype');

    // Adding Open Graph meta info
    function insert_fb_in_head() {
      global $post;
      // Use featured image if available
      if (has_post_thumbnail( $post->ID )) {
        $thumbnail_src = wp_get_attachment_image_src(get_post_thumbnail_id( $post->ID ), 'medium' );
        echo '<meta property="og:image" content="' . esc_attr( $thumbnail_src[0] ) . '"/>';
      }
    } // insert_db_in_head()
  
    // Add function to wp_head hook
    add_action( 'wp_head', 'insert_fb_in_head', 5 );