Android สอบถามการดึงรูปจาก SDCard มาแสดง นำ Codeไปจากที่นี้และติด Error ครับ

package com.application1;

import java.io.File;
import java.util.ArrayList;
import java.util.List;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.BaseAdapter;
import android.widget.GridView;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

public class Main extends Activity {
	 List <String> ImageList;
	    
	    @Override
	    public void onCreate(Bundle savedInstanceState) {
	        super.onCreate(savedInstanceState);
	        setContentView(R.layout.activity_form2);
	        
	        /*** Get Images from SDCard ***/
	        ImageList = getSD();
	        
	        // gridView1
	        final GridView gView1 = (GridView)findViewById(R.id.gridView1); 
	        	
	        gView1.setAdapter(new ImageAdapter(this,ImageList));
	        
	        // OnClick
	    }

	    private List <String> getSD()
	    {
		    List <String> it = new ArrayList <String>();
		    File f = new File ("/mnt/sdcard/picture");
		    File[] files = f.listFiles ();
		    
		    for (int i = 0; i <files.length; i++)
		    {
		    	File  file = files[i];
		    	Log.d("Count",file.getPath());
			    it.add (file.getPath());
		    }
		    return it;
	    }
	    
	    public class ImageAdapter extends BaseAdapter 
	    {
	        private Context context;
	        private List <String> lis;
	        
	        public ImageAdapter(Context c, List <String> li) 
	        {
	        	// TODO Auto-generated method stub
	            context = c;
	            lis = li;
	        }
	 
	        public int getCount() {
	        	// TODO Auto-generated method stub
	            return lis.size();
	        }
	 
	        public Object getItem(int position) {
	        	// TODO Auto-generated method stub
	            return position;
	        }
	 
	        public long getItemId(int position) {
	        	// TODO Auto-generated method stub
	            return position;
	        }
	 
			public View getView(int position, View convertView, ViewGroup parent) {
				// TODO Auto-generated method stub
				
				LayoutInflater inflater = (LayoutInflater) context
						.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
			 
			 
					if (convertView == null) {
						convertView = inflater.inflate(R.layout.showimage, null);
					}
					
					TextView textView = (TextView) convertView.findViewById(R.id.textView1);
					String strPath = lis.get(position).toString();
					
					// Get File Name
					String fileName = strPath.substring( strPath.lastIndexOf('/')+1, strPath.length() );
					textView.setText(fileName);
					
					// Image Resource
					ImageView imageView = (ImageView) convertView.findViewById(R.id.imageView1);
					Bitmap bm = BitmapFactory.decodeFile(strPath);
					imageView.setImageBitmap(bm);
			 
					return convertView;
					
			}
	    } 
	    
	    
	    @Override
	    public boolean onCreateOptionsMenu(Menu menu) {
	        getMenuInflater().inflate(R.menu.activity_main, menu);
	        return true;
	    }

	}