Your main problem is going to come from Image.FromStream. Just because the Image class has a .FromStream() method doesn't mean that any stream is a valid image, and the html response from a web server certainly would not be a valid image.
Try this: clear your cookies in your web browser then visit
(you need an account to see links)
You will be redirected to the login page. The login page is a bunch of HTML, not an image. That is why your code is failing. It is trying to visit captcha_show.phtml and download the results as an image, but since it is not logged in, it tries to convert HTML/text to an image.
I would check into using the cxWrapper.dll that I linked to you in my previous post. I will show you how to use it (with code) later.