That worked perfectly. Thank you!

Here's the code I used if it helps anyone else..

====================

Page.php
--------------------
public static $has_one = array(
   "TopImage" => "Image"
);

MyImage.php
---------------------
class MyImage extends DataObjectDecorator {
   public function GenerateTitle() {
      return "abc";
   }
}

_config.php
----------------------
DataObject::add_extension('Image', 'MyImage');