As said here, libpr0n is a library that has come with Mozilla since release 0.8. The main goal of it is to "render pornographic images in an efficient way" (and since porn image detectors aren't perfect, all sorts of other images are supported as well). Authors chose this name, because "imglib2" was sort of boring. The library supports PNG, MNG, GIF, JPEG and PPM formats.
I guess Netscape marketing department will come up with a better, more marketable name for this - and with even better reason than in case of Gecko/NGLayout. =)
Home page: http://www.libpr0n.com/