ASP, PHP, Perl .. I'd say that latter two, but as you said in another post, people are opinionated.
Just stay away from IIS (Microsoft's Internet Information Server). Very vulnerable to worms, viruses, and other attacks. You can EASILY install Apache on Windows if that is the OS you prefer to use, and Apache is much more secure.
For watermarking, look at ImageMagick (yes, that is the correct spelling) which runs on Windows and also on Unix. It MAY do what you want. For profanity filtering, roll your own should be easiest in Perl, and should not add much to your development cost. Insist on having a way for you to update the naughty word list yourself, so you don't have to pay the developer to do it every time you want to add a word.