I don't do documenation well - here's the code to use jpgraphs stuff - someone else will need to write more details here about installing it, where to put files etc. etc.

action:

class profileActions extends sfActions
{

  public function executeRegister()
  {   
      if ($this->getRequest()->getMethod() == sfRequest::POST) {
          // process form
      }
      require_once 'jpgraph/jpgraph_antispam.php';
      $antispam = new AntiSpam();
      $antispam_string = $antispam->rand(5);
      $this->getUser()->setAttribute('antispam', $antispam_string);
  }

  public function executeAntispam() {
      if (!$string = $this->getUser()->getAttribute('antispam')) {
          return sfView::NONE;
      }

      require_once 'jpgraph/jpgraph_antispam.php';
      $this->getResponse()->setContentType('image/jpeg');
      $antispam = new AntiSpam($string);
      echo $antispam->stroke();
      return sfView::NONE;
  }
}

view:

        <label>Picture</label>
        <img src="<?php echo url_for('profile/antispam'); ?>" />
        <br />

validator: myCharactersValidator.class.php

  class myCharactersValidator extends sfValidator
  {
      public function execute (&$value, &$error)
      {
          $string = sfContext::getInstance()->getUser()->getAttribute('antispam');
          if ($value != $string) {

              // generate a new image....
              require_once 'jpgraph/jpgraph_antispam.php';
              $antispam = new AntiSpam();
              $antispam_string = $antispam->rand(5);
              sfContext::getInstance()->getUser()->setAttribute('antispam', $antispam_string);

              $error = $this->getParameter('characters_error');
              return false;
          }       
          return true;
      }
  }