If you are concerned about locking people out who might not want to or are not able to receive cookies then you should equally be concerned about those people who you are deliberately excluding by using "image verification" (Known as CAPTCHA to most of us.) CAPTCHA is documented as being inaccessible to a significant proportion of the population, and is also not as secure as many people seem to think they are. To be honest the inaccessibility issue should make most people think twice about using them these days as a number of countries have laws requiring equality of access to web sites.

/J\