Libpng is avilable from these sites:
http://libpng.sourceforge.net/ http://www.mirror.ac.uk/sites/ftp.libpng.org/pub/png/libpng.html ftp://swrinde.nde.swri.edu/pub/png/src/libpng.1.2.1.tar.gz
To install, pick up the sources and then:
% gunzip -c libpng-1_2_1_tar.gz | tar xf - % ln -s libpng-1.2.1 libpng % cd libpng % cp scripts/makefile.linux makefile
Libpng has no configure script so you have to do some work by hand. Edit makefile, change prefix to be /home/joe/local and any other places - some files point to ../zlib others use /usr/local/lib and /usr/local/include. On HP-UX this is trickier. CFLAGS has to match the definition for zlib.
Now build using the edited makefile:
% make % make install % cd ..