Ive also seen ones where theres a small picture in the top right and you have to drag the puzzle pieces to make the picture. I think both of these would be great and are pretty hard for robots to solve. I actually wonder if the current PMC ones are easier to solve by bots than humans some of the time lol
but the answer can be an odd number. They arent symetrical. I understand its a minor problem but also, captcha are quite easy to implement (im a web developer myself). I know its a niggle and not necessarily a priority but i feel if it frustrates me enough it must frustrate others as well, hense me asking