Jetpack: how to define a custom fallback image

in WordPress

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.