hpng does not include the test images mentioned in the cabal file

I tried to install this library using cabal-dev, and this is what I got:
$ cabal-dev add-source hpng
Building source dist at hpng for pngload-0.1
Distribution quality warnings:
When distributing packages it is encouraged to specify source control
information in the .cabal file using one or more 'source-repository' sections.
See the Cabal user guide for details.
Warning: Cannot run preprocessors. Run 'configure' command first.
Building source dist for pngload-0.1...
cabal: test_images/mustang.png: does not exist

It seems that some images are referenced in the cabal file but they are not in the repo. This commit includes the image taken from the source distributable of pngload.