IMage verification is a very bad protection measure. Not only can many bots recognize the characters accurately 90% of the time (and failures don't really matter - they can just try again), but they leave out a whole possible group of members. If your eyesight is not perfect, you may not be able to recognize the characters and fail where a bot would succeed. Or in many cases nowadays, even if your vision is perfect, you'll still only enter the right characters 50% of the time.
A (slightly) better method would be to require forum activity before creating new threads. Additionally, new posts (by new members) might require acceptance by the administration. This would require moderators to act much more quickly, but at least it would prevent people from seeing the spam. (It could be circumvented... by using an actual human. However, at the point that a human is required, your CAPTCHA is already successful.)