CSS

background image css fill screen

.Background {
  background-image: url("Your picture name.jpg");
  background-repeat: no-repeat;
  height: 100%;
  background-position: center;
  background-size: cover;
}

css full cover background image

body { 
  background: url(path/to/bg-image.jpg) no-repeat center center fixed;
  background-size: cover;
}

backgroung full screen

'use strict'
const Game = new Phaser.Game(1000, 500, Phaser.AUTO, 'game-canvas', { preload, create, update })

function preload() {
Game.load.image("background","img/opa.jpg")

}

function create() {
   Game.add.sprite(180,35,"background")
    
}

function update() {

}