Hi,
Web.Config Configuration
You will need to add the following handler to <httpHandlers> in <system.web> section of your Web.Config.
<add verb="GET" path="CaptchaImage.axd" type="MSCaptcha.CaptchaImageHandler, MSCaptcha"/>
And add the following handler to <handlers> in <system.webServer> section of your Web.Config.
<add name="CaptchaImage" verb="GET" path="CaptchaImage.axd" type="MSCaptcha.CaptchaImageHandler, MSCaptcha"/>