I'm not sure what they're called, but I'd prefer people posting comments to also type in a number/characters that are displayed in a randomly generated graphical image.

I'm referring to something [url=http://www.russellnewquist.net/index.php?title=the_only_easy_day_was_yesterday&more=1&c=1&tb=1&pb=1#comments] like this [/url]

Could somebody point me in the right direction please.