Get Exif of jpg using ExifInterface, and thumbnail inside the JPEG.

android.media.ExifInterface is a class for reading and writing Exif tags in a JPEG file. The constructor ExifInterface(String filename) reads Exif tags from the specified JPEG file.

The method hasThumbnail() check if the JPEG file has a thumbnail. And the method getThumbnail() returns the thumbnail inside the JPEG file, or null if there is no thumbnail.

Refer to the last example "Update Gallery photos in background thread"; getExif() method is added in PhotoItem class of AndroidGalleryActivity.java, To check and load thumbnail. If it has thumbnail in the JPG file, the original runResize() will be skipped. With this implementation, the display time is much shorten for large picture.

package com.AndroidGallery;

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

import android.R.color;
import android.app.Activity;
import android.app.Dialog;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.media.ExifInterface;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.view.Gravity;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.webkit.MimeTypeMap;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.Gallery;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

public class AndroidGalleryActivity extends Activity {
 
 Gallery photoBar;
 
 Button buttonOpenDialog;
 Button buttonUp, buttonSelectFolder;
 TextView jpgList;
   
 String KEY_TEXTPSS = "TEXTPSS";
 static final int CUSTOM_DIALOG_ID = 0;
 
 ListView dialog_ListView;
 
 File root;
 File curFolder;
 
 private List<String> fileList = new ArrayList<String>();
 
 class PhotoItem{
  
  ExifInterface exifInterface = null;
  boolean exifHasThumbnail = false;
  
  String bitmapImageSrc;
  Bitmap bitmapImage = null;
  
  final static int itemWidth = 150;
  final static int itemHeight = 150;
  
  Handler handler;
  
  public PhotoItem(String src){
   bitmapImageSrc = src;
   StratBackgroundProcess();
   
   bitmapImage = BitmapFactory.decodeResource(getResources(), android.R.drawable.ic_menu_gallery);
  }
  
  public Bitmap getImage(){
   return bitmapImage; 
  }
  
  private void runResize(){
   BitmapFactory.Options bmpFactoryOptions = new BitmapFactory.Options();
   bmpFactoryOptions.inJustDecodeBounds = true;

   Bitmap bitmap = BitmapFactory.decodeFile(bitmapImageSrc, bmpFactoryOptions);
   
   int heightRatio = (int)Math.ceil(bmpFactoryOptions.outHeight/(float)itemHeight);
   int widthRatio = (int)Math.ceil(bmpFactoryOptions.outWidth/(float)itemWidth);
   
   if (heightRatio > 1 || widthRatio > 1)
   {
    if (heightRatio > widthRatio){
     bmpFactoryOptions.inSampleSize = heightRatio; 
    } else {
     bmpFactoryOptions.inSampleSize = widthRatio;  
    }  
   }
             
   bmpFactoryOptions.inJustDecodeBounds = false;
   
   bitmapImage = BitmapFactory.decodeFile(bitmapImageSrc, bmpFactoryOptions);

  }
  
  private void getExif(){
   
   try {
    exifInterface = new ExifInterface(bitmapImageSrc);
    if(exifInterface.hasThumbnail()){
     byte[] thumbnail =  exifInterface.getThumbnail();
     bitmapImage = BitmapFactory.decodeByteArray(thumbnail, 0, thumbnail.length);
     exifHasThumbnail = true;
    }
   } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }

  }
  
  private void StratBackgroundProcess(){
   Runnable runnable = new Runnable(){

    @Override
    public void run() {

     getExif();
     if(!exifHasThumbnail){
      runResize();
     }
     
     handler.post(new Runnable(){

      @Override
      public void run() {
       myPhotoBarAdapter.notifyDataSetChanged();
      }});
     
    }
    
   };
   
   handler = new Handler();
   new Thread(runnable).start();
  }
  
 }
 
 PhotoBarAdapter myPhotoBarAdapter;
 
 public class PhotoBarAdapter extends BaseAdapter {
  
  Context context;
  ArrayList<PhotoItem> arrayPhotoItem;

  PhotoBarAdapter(Context c){
      context = c;
      arrayPhotoItem = new ArrayList<PhotoItem>();    
  }
  
  public void clear(){
   arrayPhotoItem.clear();
  }
  
  public void addPhotoItem(PhotoItem item){
   arrayPhotoItem.add(item);    
  }
  
  @Override
  public int getCount() {
   return arrayPhotoItem.size();
  }

  @Override
  public Object getItem(int position) {
   return arrayPhotoItem.get(position);
  }

  @Override
  public long getItemId(int position) {
   return position;
  }

  @Override
  public View getView(int position, View convertView, ViewGroup parent) {
   
   LinearLayout viewLayout = new LinearLayout(context);
   viewLayout.setLayoutParams(new Gallery.LayoutParams(200, 200));
   viewLayout.setGravity(Gravity.CENTER);
   viewLayout.setBackgroundColor(color.background_light);
   
   ImageView imageView;
   imageView = new ImageView(context);
   imageView.setLayoutParams(new Gallery.LayoutParams(150, 150));
   imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
   imageView.setImageBitmap(arrayPhotoItem.get(position).getImage());
   //return imageView;
   
   viewLayout.addView(imageView);
   return viewLayout;
  }
  
 }
 
 /** Called when the activity is first created. */
 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  
  photoBar = (Gallery)findViewById(R.id.photobar);
  myPhotoBarAdapter = new PhotoBarAdapter(this);
  photoBar.setAdapter(myPhotoBarAdapter);
   
  jpgList = (TextView)findViewById(R.id.jpglist);
   
  buttonOpenDialog = (Button)findViewById(R.id.opendialog);
  buttonOpenDialog.setOnClickListener(new Button.OnClickListener(){
   @Override
   public void onClick(View arg0) {
    showDialog(CUSTOM_DIALOG_ID); 
   }});
   
  root = new File(Environment
    .getExternalStorageDirectory()
    .getAbsolutePath());
   
  curFolder = root;
  //ListJpgInFolder(curFolder);
  preparePhotoBarInFolder(curFolder);
   
 }
  
 @Override
 protected Dialog onCreateDialog(int id) {
   
  Dialog dialog = null;
   
  switch(id) {
  case CUSTOM_DIALOG_ID:
   dialog = new Dialog(AndroidGalleryActivity.this);
   dialog.setContentView(R.layout.dialoglayout);
   dialog.setTitle("Select JPG");
    
   dialog.setCancelable(true);
   dialog.setCanceledOnTouchOutside(true);
    
   buttonUp = (Button)dialog.findViewById(R.id.up);
   buttonUp.setOnClickListener(new OnClickListener(){
     
    @Override
    public void onClick(View v) {

     ListDir(curFolder.getParentFile()); 
    }});
   
   buttonSelectFolder = (Button)dialog.findViewById(R.id.selectfolder);
   buttonSelectFolder.setOnClickListener(new OnClickListener(){
    
   @Override
   public void onClick(View v) {

    Toast.makeText(AndroidGalleryActivity.this,
      curFolder + " selected",
      Toast.LENGTH_LONG).show();
    dismissDialog(CUSTOM_DIALOG_ID);
    
    //ListJpgInFolder(curFolder);
    preparePhotoBarInFolder(curFolder);

   }});
   
    
   //Prepare ListView in dialog
   dialog_ListView = (ListView)dialog.findViewById(R.id.dialoglist);
    
   dialog_ListView.setOnItemClickListener(new OnItemClickListener(){
     
    @Override
    public void onItemClick(AdapterView<?> parent, View view,
      int position, long id) {
     File selected = new File(fileList.get(position));
     if(selected.isDirectory()){
      ListDir(selected);  
     } 
    }});
        
   break;
  }
  return dialog; 
 }
  
 @Override
 protected void onPrepareDialog(int id, Dialog dialog, Bundle bundle) {

  super.onPrepareDialog(id, dialog, bundle);
   
  switch(id) {
  case CUSTOM_DIALOG_ID:
   ListDir(curFolder);
   
   break; 
  } 
 }
  
 void ListDir(File f){
   
  if(f.equals(root)){
   buttonUp.setEnabled(false); 
  }else{
   buttonUp.setEnabled(true); 
  }
   
  curFolder = f;
  buttonSelectFolder.setText("Select Folder " + curFolder);
   
  File[] files = f.listFiles();
  fileList.clear();
  for (File file : files){
   if(file.isDirectory()){
    fileList.add(file.getPath()); 
   }else{
    Uri selectedUri = Uri.fromFile(file);
    String fileExtension
     = MimeTypeMap.getFileExtensionFromUrl(selectedUri.toString());
    if(fileExtension.equalsIgnoreCase("jpg")){
     fileList.add(file.getName()); 
    } 
   } 
  }
   
  ArrayAdapter<String> directoryList
   = new ArrayAdapter<String>(this,
     android.R.layout.simple_list_item_1, fileList);
  dialog_ListView.setAdapter(directoryList); 
 }
 
 private void preparePhotoBarInFolder(File folder){
  
  String jpgs = "JPG fiels in folder " + folder.getAbsolutePath() + "\n\n";
  
  File[] files = folder.listFiles();
  
  myPhotoBarAdapter.clear();
  
  for (File file : files){
   if(!file.isDirectory()){
    Uri selectedUri = Uri.fromFile(file);
    String fileExtension
     = MimeTypeMap.getFileExtensionFromUrl(selectedUri.toString());
    if(fileExtension.equalsIgnoreCase("jpg")){
     jpgs += file.getAbsolutePath() + "\n";
     
     PhotoItem pItem = new PhotoItem(file.getAbsolutePath());
     myPhotoBarAdapter.addPhotoItem(pItem);
     
     //Forcer myPhotoBarAdapter update when each photos added.
     myPhotoBarAdapter.notifyDataSetChanged();
    }  
   }  
  }
  
  //Forcer myPhotoBarAdapter update again even without photo.
  myPhotoBarAdapter.notifyDataSetChanged();
  
  jpgList.setText(jpgs); 
 }
  
}

Next:
- Implement AdapterView.OnItemClickListener for Gallery