Mod that allows guest posting via Image check

I have a bot farm that comes to my site about every hour or so and tries to post spam messages. They used to do it on one forum section that I had because I had "Guest" posting enabled so people who wanted to sign up for the guild could, without having to register first.
Well now I want to see if there is a way or mod that allows you to add a recognize the image authentication type check so i can turn guest posting back on.
Basically you see the image, enter the letters or numbers, and then you are allowed to post your message.
Oh I am using 1.40 with no plans to upgrade to 1.41 yet.
Thanks,
-jack
Well now I want to see if there is a way or mod that allows you to add a recognize the image authentication type check so i can turn guest posting back on.
Basically you see the image, enter the letters or numbers, and then you are allowed to post your message.
Oh I am using 1.40 with no plans to upgrade to 1.41 yet.
Thanks,
-jack