'Request a CAPTCHA image given an `iden`.
An iden is given as the `captcha` field with a `BAD_CAPTCHA`
error, you should use this endpoint if you get a
`BAD_CAPTCHA` error response.
Responds with a 120x50 `image/png` which should be displayed
to the user.
The user\'s response to the CAPTCHA should be sent as `captcha`
along with your request.
To request a new CAPTCHA,
use [/api/new_captcha](#POST_api_new_captcha).'

@allow_oauth2_access
@api_doc(api_section.captcha, uri='/captcha/{iden}')
def GET_captchaimg(self, iden):

    image = captcha.get_image(iden)
    f = StringIO.StringIO()
    image.save(f, 'PNG')
    response.content_type = 'image/png;'
    return f.getvalue()
