[android help] android:background don't work except for overview

I work on an Android application, and I just try to put a background on an empty activity by way of welcome in the application. In the overview of the xml file, the image is displayed, but when i try the app on the emulator or on my phone, she's not displayed. They have no error, I don't understand. Can someone help me please ?

public class MainActivity extends Activity {

private SQLiteDatabase db;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Intent i = new Intent(this, ListeContact.class);
    try
    {
        Thread.sleep(3000);
    }
    catch (InterruptedException e)
    {
        e.printStackTrace();
    }
    startActivity(i);
    this.finish();
}
}

activity_main.xml :

android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="@drawable/fond">

P.S. My image named fond.png is on 5 folder named drawable-hdpi, drawable-ldpi, drawable-mdpi, drawable-xdpi and drawable-xxdpi.

.

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