The ebuild fails to create a system link from /usr/lib/libpng.so.3 to /usr/lib/libpng.so.3.1.2.3 which causes a lot of progs not to run.
the solution, apparently, is to do this: emerge -c libpng emerge libpng
portage related bug that it fails mtimes when you have "autoclean" enabled. emerge unmerge libpng; emerge libpng Will remove all instances and then install it "clean" and that will work.