[Jetpack] Add a default Image Open Graph tag only on the home page (because Jetpack doesn’t add an image tag on the home page)

plugin.php

<?php
function fb_home_image() {
    $fb_home_img = 'YOUR_IMAGE_URL';
    $fb_home_img_output = sprintf( '<meta property="og:image" content="%s" />', esc_attr( $fb_home_img ) );
    if ( is_home() )
        echo $fb_home_img_output;
}
add_action( 'wp_head', 'fb_home_image' );