Typically you'd use a third party control to generate this stuff, there are freeware ones. The is one available in the CodeProject: "
A CAPTCHA Server Control for ASP.NET[
^]"
Just returning pre-defined static images is no good, a bot will be able to parse the html and know what it is, so it all needs to be done on the fly and preferable with random "noise" to prevent OCR training.
Nice to see you have such rarefied tastes in pr0n BTW :). A better stratgey would be to take a screen shot cut out the
relevant bits and upload to
http://imgur.com/[
^]. This has two benefits: people answering questions won't get sacked from downloading porn at work, and you won't get banned from the CodeProject ;)