var gulp = require('gulp');
var imagemin = require('gulp-imagemin');
gulp.task('default', function () { console.log('Hello Gulp!') });
gulp.task('imagemin', function() {
   var img_src = 'src/images/**/*', img_dest = 'build/images';

   gulp.src(img_src)
   .pipe(changed(img_dest))
   .pipe(imagemin())
   .pipe(gulp.dest(img_dest));
});

[11:35:12] Using gulpfile C:xampphtdocsworkgulpfile.js
[11:35:12] Starting 'imagemin'...
[11:35:12] 'imagemin' errored after 10 ms
[11:35:12] ReferenceError: changed is not defined
    at Gulp.<anonymous> (C:xampphtdocsworkgulpfile.js:8:10)
    at module.exports (C:xampphtdocsworknode_modulesorchestratorlibrunTask.js:34:7)
    at Gulp.Orchestrator._runTask (C:xampphtdocsworknode_modulesorchestratorindex.js:273:3)
    at Gulp.Orchestrator._runStep (C:xampphtdocsworknode_modulesorchestratorindex.js:214:10)
    at Gulp.Orchestrator.start (C:xampphtdocsworknode_modulesorchestratorindex.js:134:8)
    at C:Users58_userAppDataRoamingnpmnode_modulesgulpbingulp.js:129:20
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)
    at Module.runMain (module.js:607:11)
    at run (bootstrap_node.js:418:7)

var gulp = require('gulp');
var imagemin = require('gulp-imagemin');
gulp.task('default', function () { console.log('Hello Gulp!') });
gulp.task('imagemin', function() {
   var img_src = 'src/images/**/*', img_dest = 'build/images';

   gulp.src(img_src)
   .pipe(imagemin())
   .pipe(gulp.dest(img_dest));
});

npm i gulp-changed --save-dev

var changed = require('gulp-changed');