My latest Captcha challenge involved identifying taxis. Well, I've seen one or two American films (movies) in which the taxis were yellow, so I made a guess and punted that the yellow cars were taxis and those of other colours weren't. It let me in. But this is no way to run an international site. A large proportion of users are not from America, have never been to America, and might not even recognize these pictures as American: they certainly don't know whether yellow=taxi is something that's universal across the US.
Please make the site more accessible to everyone in the world: this kind of thing isn't acceptable.