The most common way to achieve those features is to implement some sort of interface, usually in JSON or XML.
To do this, you either use a plugin (http://wordpress.org/plugins/json-rest-api/ <= Will probably part of core in WP 4.0) or do the AJAX function yourself.
An example would be:
add_action( 'wp_ajax_get_thumbnail', 'ajax_get_thumbnail' );
add_action( 'wp_ajax_nopriv_get_thumbnail', 'ajax_get_thumbnail' );
function ajax_get_thumbnail() {
$post_id = $_GET['post_id'];
$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'post-thumbnail' );
echo json_encode($thumb);
die(); // this is required to return a proper result
}
The call for that action would be: http://www.yourwebhost.com/wp-admin/admin-ajax.php?action=get_thumbnail&post_id=15
Please mind, that the 15 is just an example. You would get a json object in return, that contains witdh, height and the url to the post thumbnail.