No image when share website

Nope, that is what I would have thought would be the best way.

You might want to look here
http://jetpack.me/2013/07/12/add-a-default-image-open-graph-tag-on-home-page/

If you use a child theme, in your child themes functions.php file you can add this code:

function fb_home_image( $tags ) {
    if ( is_home() || is_front_page() ) {
        // Remove the default blank image added by Jetpack
        unset( $tags['og:image'] );

        $fb_home_img = 'YOUR_IMAGE_URL';
        $tags['og:image'] = esc_url( $fb_home_img );
    }
    return $tags;
}
add_filter( 'jetpack_open_graph_tags', 'fb_home_image' );

Obviously, where it says “YOUR_IMAGE_URL” you will have to copy the file path for the image you want to use, and paste it between those code ticks.