Here is a first silverstripe tutorial 

  1. PHP :
    1. create a new page type for holding coniguration values. add any field you like
       (file : mysite/code/ConfigPage.php ). Here we have added an image
      <?php
      class ConfigPage extends SiteTree {
       
       static $icon = "tutorial/images/treeicons/news";
       static $db = array( );
       static $has_one = array( 'Photo1' => 'Image'   ); 
       
         function getCMSFields() {
         $fields = parent::getCMSFields();
          $fields->addFieldToTab('Root.Content.Main', new ImageField('Photo1'), 'Content');
       return $fields;
         }
         static $defaults = array(
         'ShowInMenus' => false
      );
      }
      class ConfigPage_Controller extends ContentController {
       function init() {
        parent::init();
      }
      ?>

    2. add the config function in your Page Controller ( mysite/code/Page.php) :
      function Config( ) {
        $config = DataObject::get_one("ConfigPage");
        return  $config;
      }
  2. TEMPLATE : place the values in your templates where desired
    example :
    <% control Config %>
       $Title
        $Photo1
      <% end_control %>
  3. WEB : rebuild database (/db/build?flush=1)
  4. ADMIN : create a new Config Page and set up values