You win the nitpick of the year award :-) Actually there is a plane and a snowflake which share this property with the animals. But the set of images seems to

You win the nitpick of the year award :-) Actually there is a plane and a snowflake which share this property with the animals. But the set of images seems to be small - I got quite a few repetitions with just a few retries, so you could probably just hardcode it. Which doesn't matter much - my feeling is that this test is rather intended to put humans into the right mood than to really deter bots.