If you’re not thrilled with the provided options for a default avatar, you can use this to quickly add your own image to the list.
add_filter( 'avatar_defaults', 'addavatar' );
function addavatar ( $avatar_defaults ) {
$avatar_defaults[ 'http://placekitten.com/512/512' ] = 'Placekitten';
return $avatar_defaults;
}