Drupal media module fetches thumbnails for videos and stores them locally. Here is a snippet how you can access the thumbnail and render it using an image style.
$media = field_get_items('node', $node, 'field_media');
if (!empty($media)) {
$media = $media[0]['file'];
if ($media->type == 'video') {
$video = file_stream_wrapper_get_instance_by_uri($media->uri);
$thumbnail = $video->getLocalThumbnailPath();
$image_path = file_create_url(image_style_url('og_image', $thumbnail));
}
}