images: Optimize PNG images further with optipng and advpng. (Closes #748)

The commands used for recompression were:

    optipng -zc1-9 -zm1-9 -zs0-3 -f0-5 *.png
    advpng -z3 *.png
    advpng -z4 -i500 *.png

Note that advpng uses different algorithms when recompressing the image. The
3 refers to 7zip's deflate implementation, while 4 refers to Google's
zopfli.  For zopfli, the argument to -i specifies how many iterations to
run.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>