Talking about captchas being more or less unreadable, the funniest captcha I have ever seen, was a captcha of three (rotated) letters, where the third was painted in such a way, that it didn't fit into the picture, i.e. the third letter was intentionally clipped off somewhere in the middle, sometimes in the first third ;) Then there are captchas, where you have to tell, in which picture(s) women are, similar to the idea with the animals, they seemed quite secure, but it should be possible to adapt captcha software to that.
