为什么点击后文字变成不开心了，但图片却不变化？

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:orientation="vertical" >

    <TextView 

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:text="你今天开心吗？"/>

    <ImageView 

        android:id="@+id/imageView"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:src="@drawable/happy"

        />

    <ToggleButton 

        android:id="@+id/tb"

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:checked="true"

        android:textOn="开心"

        android:textOff="不开心"

        />

</LinearLayout>

package com.yangxj.togglebutton;

import android.app.Activity;

import android.hardware.SensorEventListener;

import android.os.Bundle;

import android.view.Menu;

import android.view.MenuItem;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.ImageView;

import android.widget.RadioGroup;

import android.widget.RadioGroup.OnCheckedChangeListener;

import android.widget.Toast;

import android.widget.ToggleButton;

public class MainActivity extends Activity {

private ImageView iv;

private ToggleButton tb;

private toggleButtonListener tbl;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_togglebutton);

initview();

setListener();

}

private void setListener() {

// TODO Auto-generated method stub

tb.setOnClickListener(tbl);

}

private void initview() {

// 初始化控件

iv = (ImageView) findViewById(R.id.imageView);

tb = (ToggleButton) findViewById(R.id.tb);

}

class toggleButtonListener implements OnClickListener {

@Override

public void onClick(View v) {

// TODO Auto-generated method stub

if (tb.isChecked()) {

Toast.makeText(MainActivity.this, tb.getText().toString(), 100).show();

iv.setImageResource(R.drawable.happy);

} else {

Toast.makeText(MainActivity.this, tb.getText().toString(), 100).show();

iv.setImageResource(R.drawable.unhappy);

}

}

}

}