public View getGroupView(int groupPosition, boolean isExpanded, View convertView,
                ViewGroup parent) {
            TextView textView = getGenericView();
            textView.setText(getGroup(groupPosition).toString());
            
//그룹에 child가 없으면 정해진 이미지를 찍기 위해서 카운트0일때 expand
            if(getChildrenCount(groupPosition) == 0){
             getExpandableListView().expandGroup(groupPosition);
            }
            return textView;
        }

<selector xmlns:android="http://schemas.android.com/apk/res/android">
 <item android:state_expanded="true" android:state_empty="true" android:drawable="@drawable/icon48x48_2"/>
    <item
        android:state_expanded="true"
        android:drawable="@drawable/expander_ic_maximized" />
    <item
        android:drawable="@drawable/expander_ic_minimized" />
</selector>

public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Set up our adapter
        mAdapter = new MyExpandableListAdapter();
        setListAdapter(mAdapter);
        registerForContextMenu(getExpandableListView());
        StateListDrawable groupIndicator = (StateListDrawable)getResources().getDrawable(R.drawable.group_indicator_01);
        getExpandableListView().setGroupIndicator(groupIndicator);
        
    }

지금 보시는 예제에 부분부분만 제가 드린대로 바꿔보세요...
그리고 저 인디케이터 이미지는 SDK에서 찾아서 복사해서 실행하셔야 합니다.