如何给Android中的按钮添加图片功能

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:Android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:gravity="center"
    android:background="@drawable/app_icon"
    android:orientation="vertical" >
    <Button
      android:id="@+id/startOrder_btn"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:background="@drawable/apply"
      android:textSize="25sp" >
    </Button>
    <Button
      android:id="@+id/managerOrder_btn"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:background="@drawable/apply"
      android:textSize="25sp" >
    </Button>
</LinearLayout>
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在drawable-xxhdpi中添加图片
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LayoutInflater inflater = LayoutInflater.from(mMainActivity);
  View layout=inflater.inflate(R.layout.my_login,null);
  ContextThemeWrapper contextThemeWrapper = new ContextThemeWrapper(MainActivity.this, 2131099656);
  AlertDialog.Builder builder =new AlertDialog.Builder(contextThemeWrapper);
  builder.setView(layout);
  builder.setCancelable(false);
  builder.create().show();
  Button startOrder_btn=(Button) layout.findViewById(R.id.startOrder_btn);
  Button managerOrder_btn=(Button) layout.findViewById(R.id.managerOrder_btn);
  startOrder_btn.setOnClickListener(new OnClickListener(){
  public void onClick(View v) {
  //加上你要实现的代码
  YSDKApi.login(ePlatform.QQ);
  }
  });
  managerOrder_btn.setOnClickListener(new OnClickListener(){
  public void onClick(View v) {//加上你要实现的代码
  YSDKApi.login(ePlatform.WX);
  }
  });