It would be great if Security images were included into CB. They can form part in the registration process, Login-Module, Forum Posts, comments, contact information, etc.
You need to do two things. First, you take the code that produces the image and the corresponding cipher or key and put that code inside funtion showregistration (see the plugin apu docs)
Then you take function onbeforeuserregistration and put the code that checks the image there. If the image does not match to the userinput, you return error and the user is not created.
If you know the security images code this should be mostly copy-pasteing.
Another option is to call securityimages functions from the plugin.