gruntfile.js

/*
 run this with one of these options:
  "grunt" alone creates a new, completed images directory
  "grunt clean" removes the images directory
  "grunt responsive_images" re-processes images without removing the old ones
*/

module.exports = function(grunt) {

  grunt.initConfig({
    responsive_images: {
      dev: {
        options: {
          sizes: [
            {width:1600, suffix: '_large_1x', quality:60},
            {width:800, suffix: '_medium_1x', quality:70},
            {width:550, suffix: '_small_1x', quality:100}
          ]          
        },

        /*
        You don't need to change this part if you don't change
        the directory structure.
        */
        files: [{
          expand: true,
          src: ['*.{gif,jpg,png}'],
          cwd: 'img',
          dest: 'images/'
        }]
      }
    },

    /* Clear out the images directory if it exists */
    clean: {
      dev: {
        src: ['images'],
      },
    },

    /* Generate the images directory if it is missing */
    mkdir: {
      dev: {
        options: {
          create: ['images']
        },
      },
    },

    /* Copy any images we don't want to resize */
    copy: {dev: {files: [{expand: true, src: 'imgs/fixed/*.{gif,jpg,png}', dest: 'images/'}]},},});
  
  grunt.loadNpmTasks('grunt-responsive-images');
  grunt.loadNpmTasks('grunt-contrib-clean');
  grunt.loadNpmTasks('grunt-contrib-copy');
  grunt.loadNpmTasks('grunt-mkdir');
  grunt.registerTask('default', ['clean','responsive_images']);

};