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: {
          engine: 'im',
          sizes: [
            /*
            Change these:

            width: ,
            suffix: ,
            quality:
            */
            {name: 'small',
              width: 320,
              height: 240,
              separator: '_',
              quality: 80
            },{
              name: 'medium',
              width: 800,
              separator: '_',
              suffix: '_1x',
              quality: 60
            },{
              name: 'large',
              width: 1600,
              separator: '_',
              suffix: '_2x',
              quality: 30
            }
          ]
        },

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

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

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

    /* Copy the "fixed" images that don't go through processing into the images/directory */
    copy: {
      dev: {
        files: [{
          expand: true,
          src: 'images_src/fixed/*.{gif,jpg,png}',
          dest: 'assets/images_optimized/'
        }]
      },
    },
  });

  grunt.loadNpmTasks('grunt-responsive-images');
  grunt.loadNpmTasks('grunt-contrib-clean');
  grunt.loadNpmTasks('grunt-contrib-copy');
  grunt.loadNpmTasks('grunt-mkdir');
  grunt.registerTask('default', ['clean', 'mkdir', 'copy', 'responsive_images']);

};