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.
Note
For some reason you may need to use --optipngpath=<OPTIPNG_PATH> in order to make glue work with optipng on Windows. More info.
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