关于SurfferView的图片问题

我想在SufferView 中弄一张我的背景图片，可是我每次弄上去的只是贴在背景图片上。
想问问怎么把一张Drable中的图片弄成背景。。谢谢了。

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
sv = (SurfaceView) findViewById(R.id.surfaceView1);
layout = (LinearLayout) findViewById(R.id.LinearLayout1);
map = BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);
holder = (SurfaceHolder) sv.getHolder();

    holder.addCallback(new Callback() {

        @Override
        public void surfaceDestroyed(SurfaceHolder holder) {

        }

        @Override
        public void surfaceCreated(SurfaceHolder holder) {

             Canvas canvas = holder.lockCanvas();

             canvas.drawBitmap(map,0, 0, null);
            holder.unlockCanvasAndPost(canvas);
        }

        @Override
        public void surfaceChanged(SurfaceHolder holder, int format, int width,
                int height) {
            // TODO Auto-generated method stub

        }
    });


}

}
不知道这个为甚不可以。