Gruntfile.js

module.exports = function(grunt) {

  grunt.initConfig({
    responsive_images: {
	dev: {
        options: {
          engine: 'im',
          sizes: [
			{ name: "_350", quality: 60, width: 350, separator: '' },
            { name: "_550", quality: 60, width: 550, separator: '' },
            { name: "_860", quality: 60, width: 860, separator: '' }
			]
        },
        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 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: 'images/',
          flatten: true,
        }]
      },
    },

  });

  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']);

};