@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Button btn=(Button)findViewById(R.id.btn);
    final RelativeLayout background = (RelativeLayout) findViewById(R.id.back);
    Resources res = getResources();
    final TypedArray myImages = res.obtainTypedArray(R.array.myImages);
    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            final Random random = new Random();
            int randomInt = random.nextInt(myImages.length());
            int drawableID = myImages.getResourceId(randomInt, -1);
            background.setBackgroundResource(drawableID);
        }
    });
}