OptiPNG is not a glue requirement but is hardly recommended to optimize the output PNG files to make them as small as possible. OptiPNG is a PNG optimizer that recompresses image files to a smaller size, without losing any information.
OptiPNG distributes an executable version for Windows, so you can download it from the optipng sourceforge page.
To install optipng from source, download the last available version and then you can install it doing:
$ tar zvxf optipng-0.6.5.tar.gz
$ cd optipng-0.6.5
$ ./configure
$ make
$ sudo make install