I realized that unfortunately the current illustration on the CAPTCHA page does not make sense.
I think that this is supposed to illustrate the Turing test, but the illustration misses the point because the examiner can see the subject physically in this picture. The point of the Turing test is that the examiner and the subject are kept in separate rooms so that the examiner has to distinguish a human from a computer by interacting with the subject only through a communication channel that is equally available to both a human and a computer.
I added a separator between the examiner and the two subjects claiming to be human. This may not be sufficient, but I hope that it is better than the current picture: