Android : Android - AnimationDrawable with setImageResource for no stretched image?

I have rocket_thrust.xml in res/drawable/:

<animation-list xmlns:android="http://schemas.android.com/apk/res/android"
android:oneshot="false">
<item android:drawable="@drawable/r1" android:duration="200" />
<item android:drawable="@drawable/r2" android:duration="200" />
</animation-list>

And ImageView in res/layout/activity_main.xml

<ImageView
android:id="@+id/rocket_image"
android:orientation="vertical"
android:layout_alignParentBottom="true"
android:layout_width="fill_parent"
android:layout_height="wrap_content" >
</ImageView>

MainActivity.java

AnimationDrawable rocketAnimation;

public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

ImageView rocketImage = (ImageView) findViewById(R.id.rocket_image);
rocketImage.setBackgroundResource(R.drawable.rocket_thrust);
rocketAnimation = (AnimationDrawable) rocketImage.getBackground();

showAnim();
}

public void showAnim() {
rocketAnimation.start();
}

This code is working.

But, the image is stretched. I don't want this stretching.

I changed:

rocketImage.setBackgroundResource(R.drawable.rocket_thrust);

with:

rocketImage.setImageResource(R.drawable.rocket_thrust);

for no stretching. The code is now not working.

How can I solve it?