Anything redistributed, paid or free, containing material that is someone else's intellectual property legally does actually require permission (Pernangband being one case that hit a cease and desist that I can think of straight off the top of my head). I'm not entirely clear how far "fair use" stretches, but I suspect "including someone else's material in a mod that other people can download" is probably a part of the grey area that I wouldn't want to have to fight in a courtroom.
That said, it's rare that anyone actually does anything about stuff for fan based free things, so I'd either rename it / resprite it, but leave it obvious where the inspiration came from, or use something accurate, cite and credit both on the forums and on the credits of the mod itself, and I would have thought you'd be more than safe with it.