The render() method of Zend_Captcha_Image does not return the width and height of the image captcha:

 public function render(Zend_View_Interface $view, $element = null)
{
    return '
';
}

The following code does:

 public function render(Zend_View_Interface $view, $element = null)
{
    return '
';

}

Reasoning: "Without this information in the HTML, [the browser] cannot account for the viewport space needed for the image. As a result, it happens that if the user is reading below the image, suddenly [the browser] discovers the dimensions and the whole page jumps disconcertingly." Source: http://is.gd/Da3G)