Here is what I did to make this work.
I added: public $useTemplate = "ImageGalleryPage_album"; to ImageGalleryPage.php
and changed the ImageGalleryPage.php init to look like this:
public function init()
{
parent::init();
self::prototype2jquery();
Requirements::css('image_gallery/css/ImageGallery.css');
if ($this->useTemplate == "ImageGalleryPage_album"){
if(!isset($this->urlParams['Action'])) {
if($this->SingleAlbumView()) {
die($this->renderWith(array('$this->useTemplate','Page')));
}
}
else if($this->CurrentAlbum())
$this->includeUI();
}
}
then I set the public $useTemplate = "ImageGalleryPage_album in my subclass, then it all seems to work.