Batch compression of PNG files

Gruntfile.js

var APIKEY = "OOOOOOOOOOOOOOOOOOOOOOOOOO";
module.exports = function(grunt){
    grunt.loadNpmTasks('grunt-tinypng');
    grunt.initConfig({
        tinypng: {
            options: {
                apiKey: APIKEY,
                showProgress: true,
                stopOnImageError: false
            },
            compress: {
                src: 'source/**/*.png',
                dest: 'dest/',
                expand : true
            }
        }
    })
}