This directory contains "armored detached" ascii signatures
for the libpng-1.6.4 source files.

Use this public key (which may be found at pgp.mit.edu among other places)
Just search for "libpng" and select this one:

pub  4096R/A16C640F 2013-05-31 Glenn Randers-Pehrson
     (libpng) <glennrp@users.sourceforge.net>

and put the result in your public keyring.

To verify, for example, libpng-1.6.4.tar.xz, download libpng-1.6.4.tar.xz.asc
into the directory where you put the libpng-1.6.4.tar.xz, then type 
gpg --verify libpng-1.6.4.tar.xz.asc libpng-1.6.4.tar.xz (or put the
equivalent of these two actions into your download/build script).

If you know how to do the equivalent on a non-Linux platform, do that instead.

Glenn