Using: `"grunt-contrib-imagemin" : "~0.5.0",` See https://core.trac.wordpress.org/ticket/25169

gistfile1.txt

> gifsicle@0.1.4 postinstall C:\xampp\htdocs\develop.wp.nw\node_modules\grunt-contrib-imagemin\node_modules\image-min\node_modules\gifsicle
> node lib/check.js

  gifsicle.exe: downloading [==                 ] 13% 1.3s
> jpegtran-bin@0.2.3 postinstall C:\xampp\htdocs\develop.wp.nw\node_modules\grunt-contrib-imagemin\node_modules\image-min\node_modules\jpegtran-bin
> node lib/check.js

  gifsicle.exe: downloading [===================] 100% 0.0s


✓ pre-build test passed successfully
  libjpeg-62.dll: downloading [======             ] 36% 0.7s
> optipng-bin@0.3.1 postinstall C:\xampp\htdocs\develop.wp.nw\node_modules\grunt-contrib-imagemin\node_modules\image-min\node_modules\optipng-bin
> node lib/check.js

  libjpeg-62.dll: downloading [===================] 100% 0.0s


  libjpeg-62.dll: downloading [===================] 100% 0.0s


✓ pre-build test passed successfully
  optipng.exe: downloading [===================] 100% 0.0s


✗ pre-build test failed, compiling from source...

C:\xampp\htdocs\develop.wp.nw\node_modules\grunt-contrib-imagemin\node_modules\image-min\node_modules\optipng-bin\lib\check.js:11
            throw new Error('building is not supported on ' + process.platform
                  ^
Error: building is not supported on win32
    at C:\xampp\htdocs\develop.wp.nw\node_modules\grunt-contrib-imagemin\node_modules\image-min\node_modules\optipng-bin\lib\check.js:11:19
    at ChildProcess.<anonymous> (C:\xampp\htdocs\develop.wp.nw\node_modules\grunt-contrib-imagemin\node_modules\image-min\node_modules\optipng-bin\nod
e_modules\bin-wrapper\bin-wrapper.js:174:16)
    at ChildProcess.EventEmitter.emit (events.js:98:17)
    at Process.ChildProcess._handle.onexit (child_process.js:789:12)
npm WARN optional dep failed, continuing optipng-bin@0.3.1
grunt-contrib-imagemin@0.5.0 node_modules\grunt-contrib-imagemin
├── filesize@2.0.0
grunt-contrib-imagemin@0.5.0 node_modules\grunt-contrib-imagemin
├── filesize@2.0.0
├── async@0.2.10
├── chalk@0.4.0 (has-color@0.1.4, ansi-styles@1.0.0, strip-ansi@0.1.1)
└── image-min@0.1.3 (mkdirp@0.3.5, cache-file@0.1.4, mout@0.7.1, gifsicle@0.1.4, jpegtran-bin@0.2.3)