class Page extends SiteTree {

    private static $db = array();

    private static $has_one = array(
        'SideImage'=>'Image'
    );

    public function getCMSFields() {
        $this->beforeUpdateCMSFields(function($fields) {
            $fields->addFieldToTab('Root.Images', UploadField::create('SideImage','Image for Right Panel'));
        });
        return parent::getCMSFields();
    }

}

And in config.php:
Page:
  extensions:
    - 'FluentFilteredExtension'
  translate:
    - 'SideImageID'