_config.php:
Object::add_extension('HeaderImage', 'Translatable');

CategoryPage:
class CategoryPage extends Page {

static $has_many = array(
'HeaderImages' => 'HeaderImage',
);

function getCMSFields() {

$fields = parent::getCMSFields();

    $manager = new DataObjectManager(
$this,
'HeaderImages', // relation name
'HeaderImage', // object class
HeaderImage::$field_names, // fields to show in table
HeaderImage::getCMSFields_forPopup(), // form that pops up for edit
"CategoryPageID = {$this->ID}" // a filter to only display item associated with this page
);

$manager->setAddTitle('Header');
$manager->setSingleTitle('Header Image');
$manager->setPluralTitle('Header Images');
$manager->setPopupWidth(400);
$manager->setPerPageMap(array(20,40,80));

    $fields->addFieldToTab('Root.Content.Header', $manager);
   }
}

HeaderImage.php:
class HeaderImage extends DataObject {

static $has_one = array(
'Image' => 'Image',
'Category' => 'Category',
'RootFolder' => 'Folder'
);

static $field_names = array(
'Thumbnail' => 'Image',
'Title' => 'Title',
);

function getCMSFields_forPopup() {

$fields = new FieldSet();

    $picfolderName = $this->RootFolder()->FileName;
    $picfolderName = str_replace('assets/','',$picfolderName);
   
    $fields->push(new TextField('Title', 'Title'));
    $fields->push(new TextField('Subtitle', 'Subtitle'));
    $fields->push(new TextField('Text', 'Text'));
    $fields->push(new DropdownField('Alignment','Alignment',singleton('HeaderImage')->dbObject('Alignment')->enumValues()));
    $fields->push(new TextField('Link', 'Link'));
$fields->push(new ImageField('Image', 'Image', null, null, null, $picfolderName));

return $fields;
}

   function getThumbnail()
   {
    if ($Image = $this->Image())
    {
    return $Image->CMSThumbnail();
    }
    else
    {
    return '(No Image)';
    }
   }

   public function onBeforeWrite() {
      parent::onBeforeWrite();
      $this->Locale = Translatable::get_current_locale();
   }

}