Android: Change app background image when orientation change(toggle b/w landscape and portrait) without restarting main class

Avoid main or activity class restart:

To do this note go to bin/res/AndroidManifest.xml, and add the following line under any activity where you want disable activity class refresh when orientation change.

android:configChanges="orientation|screenSize"

Change background Image:

To do this go to src/com.example.<yourproject>/<yourclass>Activity.java, for example src/com.example.firstapp/MainActivity.java, and add following function.

public void onConfigurationChanged(Configuration newConfig)
{
   super.onConfigurationChanged(newConfig);
   View mainLayout = findViewById(R.id.main_layout); // getting the layout

  // Checks the orientation of the screen
  if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE)
  {
    mainLayout.setBackgroundResource(R.drawable.my_bg_land);
  }
  else if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT)
  {
     mainLayout.setBackgroundResource(R.drawable.my_bg);
  }
}

Note:
Please note on above code we access layout by its id(main_layout). To use this we have note it on layout xml file(that is xml file for your specific activity which is under res/layout here:res/layout/activity_main.xml)

android:id="@+id/main_layout"

Reference:

Changing background of a layout at run time - http://manijshrestha.wordpress.com/2011/04/05/android-sdk-changing-background-of-a-layout-at-run-time/

Detect orientation change - http://stackoverflow.com/questions/5726657/how-to-detect-orientation-change-in-layout-in-android