Alt tags

Since there’s no helper in WP to get the alt attribute, you’d have to do something like:

attachment_id = MultiPostThumbnails::get_post_thumbnail_id( $post_type, $id, $post_id );
$alt = '';
$alt = trim( strip_tags( get_post_meta( $attachment_id, '_wp_attachment_image_alt', true ) ) );

if ( ! $alt ) {
  $attachment = get_post( $attachment_id );
  $alt = trim(strip_tags( $attachment->post_excerpt )); // Use Caption
  if ( ! $alt ) {
      $alt = trim(strip_tags( $attachment->post_title )); // Use Title
  }
}