Make homepage og:image be an image from the latest post

functions.php

<?php
function fb_home_image( $tags ) {
	if ( is_home() ) {
		$latest_post = get_posts("post_type=post&numberposts=1");
		$latest_post = $latest_cpt[0]->ID;

		if ( class_exists( 'Jetpack_PostImages' ) ) {
			$post_images = Jetpack_PostImages::get_images( $latest_post, array( 'width' => 200, 'height' => 200 ) );
			if ( $post_images && !is_wp_error( $post_images ) ) {
				$image = array();
				foreach ( (array) $post_images as $post_image ) {
					$image[] = $post_image['src'];
				}
			}
		}
		if ( empty( $image ) )
			$image = array();
		else if ( !is_array( $image ) )
			$image = array( $image );

	// Remove the default blank image added by Jetpack
		unset( $tags['og:image'] );

        $tags['og:image'] = $image;
    }
    return $tags;
}

add_filter( 'jetpack_open_graph_tags', 'fb_home_image' );