Returns URL to photo, optionally can link
        to different size images
        Size suffixes according to:
        http://www.flickr.com/services/api/misc.urls.html

        s   small square 75x75
        q   large square 150x150
        t   thumbnail, 100 on longest side
        m   small, 240 on longest side
        n   small, 320 on longest side
        -   medium, 500 on longest side
        z   medium 640, 640 on longest side
        c   medium 800, 800 on longest side
        b   large, 1024 on longest side*
        o   original image, either a jpg, gif or png, depending on source format

def _getphoto_url(self,pid,size='n'):
        """Returns URL to photo, optionally can link
        to different size images
        Size suffixes according to:
        http://www.flickr.com/services/api/misc.urls.html

        s   small square 75x75
        q   large square 150x150
        t   thumbnail, 100 on longest side
        m   small, 240 on longest side
        n   small, 320 on longest side
        -   medium, 500 on longest side
        z   medium 640, 640 on longest side
        c   medium 800, 800 on longest side
        b   large, 1024 on longest side*
        o   original image, either a jpg, gif or png, depending on source format

        """
        p=self._getphoto_information(pid)
        farm=p['farm']
        server=p['server']
        secret=p['secret']
        url="http://farm%s.staticflickr.com/%s/%s_%s_%s.jpg"\
            %(farm,server,pid,secret,size)

        return url