Progress Dialog doesn't work until thread finishes its job

Hello my friends!
I'm have a problem when working with progress dialog and thread.
I want to show all images from sdcard. Because of the long loading time, I'd like to use a progress dialog end then discard it when the loading images finishes.

However, it doesn't work as I expected. The progress dialog hibernates while the thread is running ( the loading images from sdcard and showing them to a listview ).

What's the problem here?

This is my codes:

// use button click to open images from sdcard
btnClick=(Button)findViewById(R.id.btnClick);
btnClick.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
System.gc();
// show progress dialog
showDialog(DIALOG2_KEY);

// start a new thread to load images and then discard the progress dilog
ShowImage();
}
})

// Create a new thread for loading images
public void ShowImage(){

thread = new Thread(this);
// it works for 5 seconds
try {
thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
thread.start();
}
public void run() {

// get Uris of Images from Sd card
getUri();
handler.sendEmptyMessage(0);
}
private Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {

// show images to gridview
g.setAdapter(new ImageAdapter(getApplicationContext()));

// remove Dialog
removeDialog(DIALOG2_KEY);
}
}