I think this is a bug:

function theme_lightbox2_emimage($field, $item, $formatter, $node, $view_type) {

  ...

  // Set up the title.
  $title = module_invoke('emfield', 'include_invoke', 'emimage', $item['provider'], 'image_title', $code, $item['data']);

Better is:

function theme_lightbox2_emimage($field, $item, $formatter, $node, $view_type) {

  ...

  $item_data = $item['data'];
  if (is_string($item['data'])) {
    $item_data = unserialize($item['data']);
  }

  // Set up the title.
  $title = module_invoke('emfield', 'include_invoke', 'emimage', $item['provider'], 'image_title', $code, $item_data);