gulpfile showing off ways of manipulating images and compiling sass

gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-ruby-sass');
var prefix = require('gulp-autoprefixer');
var csso = require('gulp-csso');
var imgmin = require('gulp-imagemin');
var resize = require('gulp-image-resize');
var changed = require('gulp-changed');
var parallel = require('concurrent-transform');
var os = require('os');
var rename = require('gulp-rename');

var config = {
  sass: './sass/style.scss',
  images: './assets/*.{png,jpg,jpeg}',
  build: './build'
}


gulp.task('sass', function(){
  return sass(config.sass)
  .pipe(prefix({
    browsers: ['last 2 versions'],
    cascade: true
  }))
  .pipe(csso())
  .pipe(gulp.dest( config.build + '/css'));
});

gulp.task('image-hd', function(){
  return gulp.src(config.images)
  .pipe(changed(config.build + '/assets'))
  .pipe(parallel(
    resize({
      width: 1920
    }),
    imgmin({
      progressive: true
    }),
    os.cpus().length
  ))
  .pipe(rename(function(path) { path.basename += '-hd'; }))
  .pipe(gulp.dest( config.build + '/assets'));
});

gulp.task('image-thumbnails', function(){
  return gulp.src(config.images)
  .pipe(changed(config.build + '/assets'))
  .pipe(parallel(
    resize({
      width: 150,
      height: 150,
      crop: true,
      gravity: 'Center'
    }),
    imgmin({
      progressive: true
    }),
    os.cpus().length
  ))
  .pipe(rename(function(path) { path.basename += '-thumbnail'; }))
  .pipe(gulp.dest( config.build + '/assets'));
});

gulp.task('images', ['image-hd','image-thumbnails']);

gulp.task('build', ['sass','images']);

gulp.task('watch', function(){
  gulp.watch('./css/**/*.scss', ['sass']);
});

gulp.task('default', ['sass','images']);