gruntfile.js

module.exports = function(grunt) {

  grunt.initConfig({
    image_resize: {
      options: { width: 16, height: 16, overwrite: true, upscale: true, crop: true },
      your_target: { 'www/assets/icon.png':'www/assets/icon16.png' },
      your_target_36: {
        options: { width: 36, height: 36 },
        files: { 'www/assets/icon.png':'www/assets/icon36.png' }
      }
    }
  });

  grunt.loadNpmTasks('grunt-image-resize');
  grunt.registerTask('default', ['image_resize']);
};