imagemin.js

module.exports = function(grunt) {

    grunt.initConfig({

        imagemin: {
            png: {
                options: {
                    optimizationLevel: 3
                },
                files: [
                    {
                        // Set to true to enable the following options…
                        expand: true,
                        // cwd is 'current working directory'
                        cwd: 'assets/images/',
                        src: ['**/*.png'],
                        // Could also match cwd line above. i.e. project-directory/img/
                        dest: '.assets/images/',
                        ext: '.png'
                    }
                ]
            },
            jpg: {
                options: {
                    progressive: true
                },
                files: [
                    {
                        // Set to true to enable the following options…
                        expand: true,
                        // cwd is 'current working directory'
                        cwd: 'assets/images/',
                        src: ['**/*.jpg'],
                        // Could also match cwd. i.e. project-directory/img/
                        dest: 'assets/images/',
                        ext: '.jpg'
                    }
                ]
            }
        }
    });

    grunt.loadNpmTasks('grunt-contrib-imagemin');

    grunt.registerTask('default', ['imagemin']); // execute on both .png and .jpg
    grunt.registerTask('imagepng', ['imagemin:png']); // only .png files
    grunt.registerTask('imagejpg', ['imagemin:jpg']);// only .jpg files
}