android ，gridview和simpleadapter,为什么显示的都是同一张图？

代码如下：

activity_main.xml:
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:paddingBottom="@dimen/activity_vertical_margin"
 android:paddingLeft="@dimen/activity_horizontal_margin"
 android:paddingRight="@dimen/activity_horizontal_margin"
 android:paddingTop="@dimen/activity_vertical_margin"
 tools:context=".MainActivity" 
 android:orientation="vertical">
<GridView
    android:id="@+id/gridview"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"/>

然后下面是imageview.xml

 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".activity" 
android:orientation="vertical">

<ImageView 
    android:id="@+id/image"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"/>

</LinearLayout>

下面是java代码：

package com.example.test5;

import android.os.Bundle;
 import android.widget.*;
 import java.util.*;
 import android.app.Activity;
 import android.view.Menu;

public class MainActivity extends Activity {
SimpleAdapter s=null;
GridView grid=null;
List<Map<String,Object>> list=null;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    grid=(GridView)super.findViewById(R.id.gridview);
    initAdapter();
    grid.setAdapter(s);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

public void initAdapter()
{
    Map<String,Object> map=new HashMap<String,Object>();
    list=new ArrayList<Map<String,Object>>();

    map.put("图片", R.drawable.a1);
    list.add(map);
    map.put("图片", R.drawable.a2);
    list.add(map);
    map.put("图片", R.drawable.a3);
    list.add(map);

    s=new SimpleAdapter(MainActivity.this,list,R.layout.imageview,new String[]{"图片"},new int[]{R.id.image});
}
}

要显示三张图片，可是显示的都是同一张，是最后一张图。

请问这里哪里出了错，怎么解决？