I have just done an hour on that site.
Bazile, They don't reveal their algorithms but since they let you take part as long as you get 8/15 correct in the test, then they must get multiple wrong classifications. I would presume/imagine they are doing it statistically and more than one person is being offered the same images to classify. It would then be classified as whatever the majority decide. Presumably they have run trials with students to find the optimum number needed to classify each image.