GM/Gulp

images.js

var gulp       = require('gulp');
var imagemin   = require('gulp-imagemin');
var newer      = require('gulp-newer');
var filter     = require('gulp-filter');
var merge      = require('merge2');
var rename     = require('gulp-rename');
var resize     = require('./resize-images')

var options = [
  { width: 200, upscale: false },
  { width: 400, upscale: false },
  { width: 600, upscale: false },
  { width: 800, upscale: false },
  { width: 1000, upscale: false },
  { width: 1200, upscale: false },
  { width: 1400, upscale: false },
  { width: 1600, upscale: false }
]

gulp.task('images', function() {
  var streams =  options.map(function(el) {
    return gulp.src(['./_images/**/*', '!./_images/**/*.svg'])
      .pipe(rename(function(file) {
        if(file.extname) {
          file.basename += '-' + el.width
        }
      }))
      .pipe(newer('images'))
      .pipe(resize(el))
      .pipe(imagemin())
      .pipe(gulp.dest('images'));
  });

  streams.push(gulp.src('_images/**/*')
    .pipe(newer('images'))
    .pipe(imagemin())
    .pipe(gulp.dest('images')));

  return merge(streams);

});

resize-images.js

var gm = require('gm');
var through = require('through2');

module.exports = function(el) {
  return through.obj(function(originalFile, enc, cb) {
    var file = originalFile.clone({contents: false});

    if (file.isNull()) {
      return cb(null, file);
    }

    var gmfile = gm(file.contents, file.path);
    gmfile.size(function(err, size) {

      if(typeof el !== 'undefined' && el.width < size.width) {
        gmfile
          .resize(el.width, (el.width / size.width) * size.height)
          .toBuffer(function (err, buffer) {
            file.contents = buffer;
            cb(null, file);
          });
      } else {
        // remove from stream
        cb(null, null);
      }
    });
  });
};