Uso do Grunt + Spritemisth

Gruntfile.js

'use strict';
module.exports = function(grunt) {

  grunt.initConfig({
    sprite:{
      all: {
        algorithm: 'binary-tree',
        src: 'public/r/sprites/*.png',
        destImg: 'public/r/sprites.794984.png',
        destCSS: 'public/r/css/sprites.less',
        cssFormat: 'less'
      }
    },
    watch: {
      sprite: {
        files: [
          'public/r/sprites/*.png'
        ],
        tasks: ['sprite']
      },
    },
  });

  // Load tasks
  grunt.loadNpmTasks('grunt-contrib-clean');
  grunt.loadNpmTasks('grunt-spritesmith');

  // Register tasks
  grunt.registerTask('default', [
    'clean',
    'sprite','
  ]);


};