Fix invalid PNG images to work with libpng 1.6

The PNG images of the Mirabilis skin fail to load with libpng 1.6. The
error, "IDAT: invalid distance too far back", is triggered by bad CMF
bytes in these PNG files.

I used optipng (statically linked against libpng 1.4.12) to optimize,
and thus fix, the affected images.
Merge pull request #32 from foutrelis/master

Fix invalid PNG images to work with libpng 1.6