Libpng is the official reference library for the PNG graphics format. It can be downloaded from ftp://swrinde.nde.swri.edu/pub/png/src/ and http://www.cdrom.com/pub/png/src/. Note that for MMX support on GNU/Linux and other systems using GNU C, you will need http://www.cdrom.com/pub/png/src/libpng-1.0.5-mmx-opt-gccasm.tar.gz. A version of libpng compatible with the latest GNU Ghostscript release can be found in the ghostscript directory on this site.