Erro minificar imagem

gulpfile.js

var gulp = require('gulp')
  ,imagemin = require('gulp-imagemin');

gulp.task('build-img', function() {

  gulp.src('src/img/**/*')
    .pipe(imagemin())
    .pipe(gulp.dest('src/img'));
});

resultado:

C:\Users\andre\Documents\estudo\Alura Gulp [Engenheiro Front End]\projeto>npm run gulp build-img

> projeto@1.0.0 gulp C:\Users\andre\Documents\estudo\Alura Gulp [Engenheiro Front End]\projeto
> gulp "build-img"

[00:59:05] Using gulpfile ~\Documents\estudo\Alura Gulp [Engenheiro Front End]\projeto\gulpfile.js
[00:59:05] Starting 'build-img'...
[00:59:05] Finished 'build-img' after 21 ms
[00:59:05] gulp-imagemin: Minified 0 images

package.json

{
  "name": "projeto",
  "version": "1.0.0",
  "description": "",
  "main": "gulpfile.js",
  "dependencies": {
    "gulp-imagemin": "^2.3.0"
  },
  "devDependencies": {},
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
      "gulp" : "gulp"
      },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "gulp": "^3.9.0",
    "gulp-imagemin": "^2.3.0"
  }
}