Jetpack handles Open Graph Meta Tags for you, and automatically creates a fallback Image meta tag if you haven’t inserted any image into your post.
If you’d like to define your own custom fallback image, you can use the jetpack_open_graph_image_default
filter, like so:
function jeherve_default_image() { $fallback_img = 'http://YOUR_IMG_URL'; return esc_url( $fallback_img ); } add_filter( 'jetpack_open_graph_image_default', 'jeherve_default_image' );
Note that the filter is new, and will only be added to the next Jetpack release, 3.0.