Gruntfile.js

module.exports = function(grunt) {

    grunt.initConfig({
        responsive_images: {
            dev: {
                options: {
                    engine: 'im',
                    sizes: [{
                        width: 800,
                        quality: 30
                    }, {
                        width: 400,
                        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: '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']
                },
            },
        },

    });

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

};