I have read the tutorial for creating a function to resize an image but i cannot seem to get it to work correctly. Here is my scenario:

GalleryItemHolder.ss - this is the main Holder template that will contain the resized thumbnails
GalleryItemPage.ss - this is the detail page for each of the GalleryItems - the images that are resized on the Holder page are attached so the GalleryItemPage class

I have added the following code to the GalleryItemPage.php

class GalleryItemPage extends Page {
   static $db = array(
   );
   static $has_one = array(
      'GalleryItemImage' => 'Image',
	  'GalleryItemThumb' => 'GalleryItemPage_Thumb150px'
   );

on what page do i put the following function class?

class GalleryItemPage_Thumb150px extends Image {

   function generateThumb150px($gd) {
      return $gd->resizeRatio(150,150);
   }

If I put in the GalleryItemPage.php file, it does not seem to be callable in the GalleryItemHolder.ss template - which is where i want the resized thumbnails to be generated.

I found the issue. I did not pick up on the fact that I needed to update the ClassName for all the existing images I had already uploaded in File table. Once I updated all the thumb images with the class name of GalleryItemPage_Thumb150px, the resize code worked. This is key for making this change after you've already uploaded images under the Image classname.