I saw a nice post in CodeProject (http://www.codeproject.com/KB/aspnet/CaptchaImage.aspx) regarding CAPTCHA image control. I used the same code to create a Custom Field that will work like a CAPTCHA field for SharePoint. I tested it with Custom List and Blog Post suceessfully. You need to enable the session in the Web.Config file using enableSessionState="true" as it depends heavily on session. Here is how it looks like:
Here is the screenshot of the Solution Explorer:
You can download the WSP file and the project in a zip file from here.