optimize-assets

This gulp task takes the asset output from Generate Mobile Assets in the Sketch plugin sketch-mobile-assets, optimizes them, and then sorts them in the output folders for iOS and Android that you define.

You should use this if:

You use Sketch
You are designing for iOS and Android at the same time (one file creates assets for both platforms)
You use sketch-mobile-assets to export your assets
You are familiar with the command line, NPM, and Gulp

Install gulp

Install gulp with these instructions

Install these NPM packages

npm install gulp-imagemin
npm install imagemin-pngquant
npm install merge-stream

Edit config.json and define these paths

source absolute path to your asset output folder (where you save your assets from Generate Mobile Assets)
ios absolute path to your asset directory for your iOS project
android absolute path to your asset directory for your Android project. optimize-assets will sort the files into the appropriate drawable Android folders

Now, run gulp!

In the directory that contains gulpfile.js, run gulp in the terminal