Verififying Image Characters

[rn5a]

Nowadays, while registering at a site, an image with alphanumeric
characters is displayed to users who are asked to enter those
alphanumeric characters in a textbox to avoid automated registration.

Are such images generated dynamically? Or is it something like the
sites have thousands & thousands of images with random alphanumeric
characters at their disposal? I don't think it could be the latter
though I am not sure about it. Assuming that the images with
alphanumeric characters are generated dynamically, how can this be
done using ASP?

Moreover, the text entered by users are also validated against the
text on the image. How can this be done using ASP?

Thanks......