create-phaser-app/src/assets/backgrounds/index.js

A boiler plate to start your phaser app with webpack4, babel7+ and Phaser 3

import BackgroundGradient from './game/background-with-planets.png';
import BackgroundClouds from './game/clouds.png';
import BackgroundSea from './game/mountain_and_sea.png';

export default {
  BackgroundClouds,
  BackgroundClouds,
  BackgroundSea
};