If I were you, I would first extend the ImageGalleryPage class, and have it relate to another extended ImageGalleryItem class. Your ImageGalleryItem subclass could have a Thumbnail field.
class MyGallery extends ImageGalleryPage
{
protected $itemClass = "MyImageGalleryItem";
static $has_many = array (
'MyImageGalleryItems' => 'MyImageGalleryItem'
);
}
class MyImageGalleryItem extends ImageGalleryItem
{
static $has_one = array ('Thumbnail' => 'Image');
public function getCMSFields_forPopup()
{
$f = parent::getCMSFields_forPopup();
$f->push(new ImageField('Thumbnail'));
return $f;
}
}
I'll make an update to the core to use the dynamic $itemClass property in the GalleryItems() control.
Then you just need to update your template to use the new thumbnail image.