i'm trying to get a Tab "Fotos" Wherein the enduser has the possibility to upload thumbnails and photo's. The upload module works, except every page shows the same pictures. I think it's in the fotos.php static has_one -> SiteTree, because of it stays 0 in the db after saving.

I want to have every page different pictures (eg home: 1.jpg, 2.jpg; contact: 3.jpg, 4.jpg),

Foto's.php:

<?php
/**
 * Defines the Fotos
 */
class Fotos extends DataObject {
  static $db = array(
      'SmallCapt' => 'Text',
      'LargeCapt' => 'Text'
  );
  
  static $has_one = array(
    'SmallFoto' => 'Image',
    'LargeFoto' => 'Image',
    'MySiteTree' => 'SiteTree'
  );

  function getCMSFields_forPopup() {
    $fields = new FieldSet();
     
    $fields->push( new SimpleImageField( 'SmallFoto', 'Kleine foto') );
    $fields->push( new Textfield( 'SmallCapt', 'Beschr. kleine foto' ) );
    $fields->push( new SimpleImageField( 'LargeFoto', 'Grote foto' ) );
    $fields->push( new Textfield( 'LargeCapt', 'Beschtr. grote foto' ) );
     
    return $fields;
  }
}
?>

Page.php

...
  public static $has_many = array(
    'MyFotos' => 'Fotos'
	);
  
  function getCMSFields() {
    $fields = parent::getCMSFields();
    $tablefield = new HasManyComplexTableField(
        $this,
        'MyFotos',
        'Fotos',
        array(
            'SmallFoto' => 'Kleine foto',
            'SmallCapt' => 'Beschrijving',
            'LargeFoto' => 'Grote foto',
            'LargeCapt' => 'Beschrijving'
        ),
        'getCMSFields_forPopup'
    );
         
    $fields->addFieldToTab('Root.Content.Fotos', $tablefield );
    return $fields;
  }
...

The Fotos Object needs to have a $has_one relation with Page, not SiteTree.

As an aside you may want to look at the DataObjectManager module instead of using a HasManyComplexTableField since it has good image relation manager ImageDataObjectManager