Add pngquant, it converts 24/32-bit RGBA PNGs to 8-bit palette with alpha channel preserved.

pkgs/tools/graphics/pngquant/default.nix

@@ -0,0 +1,26 @@
+{ stdenv, fetchgit, libpng }:
+
+stdenv.mkDerivation rec {
+  name = "pngquant-${version}";
+  version = "2.0.1";
+
+  src = fetchgit {
+    url = https://github.com/pornel/pngquant.git;
+    rev = "refs/tags/${version}";
+    sha256 = "00mrv9wgxbwy517l8i4n7n3jpzirjdgi0zass3wj29i7xyipwlhf";
+  };
+
+  buildInputs = [ libpng ];
+
+  preInstall = ''
+    mkdir -p $out/bin
+    export PREFIX=$out
+  '';
+
+  meta = with stdenv.lib; {
+    homepage = https://github.com/pornel/pngquant;
+    description = "pngquant converts 24/32-bit RGBA PNGs to 8-bit palette with alpha channel preserved";
+    platforms = platforms.all;
+    license = licenses.bsd2; # Not exactly bsd2, but alike
+  };
+}

pkgs/top-level/all-packages.nix

@@ -1596,6 +1596,8 @@ let
     libpng = libpng12;
   };
 
+  pngquant = callPackage ../tools/graphics/pngquant { };
+
   podiff = callPackage ../tools/text/podiff { };
 
   poedit = callPackage ../tools/text/poedit { };