LinearLayout findViewById problem - crash

You not loading a layout

you need to load on xml layout before using the findViewById

setContentView(R.layout.aLayout);

You have to set layout for your Activity from resources

setContentView(R.layout.my_layout);

Then you can call findViewById()

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.my_layout); // add this code
    LinearLayout ln = (LinearLayout) this.findViewById(R.id.linlay);
    ln.setBackgroundDrawable(getResources().getDrawable(R.drawable.wallpaper));
}

In your case you can simply set wallpaper in xml resource file by adding to LinearLayout

android:background="@drawable/wallpaper"