How to Draw a Rectangle on Image Bitmap in Imageview

I am new Android programming. I am trying to open an image from the Gallery of a phone and draw a Rectangle on top of the image I have opened. But I am able to open the image but I am not able to see the rectangle. I am using ImageView and Canvas to open image and draw. I have created a Button and used to image from Gallery. I have written the code to open and draw in onActiviesult() method.Could somebody help me?!. Thanks in advance

public class MainActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Button gal=(Button)findViewById(R.id.button1);
    gal.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            Intent gal_open=new Intent(Intent.ACTION_PICK,android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
            startActivityForResult(gal_open,1);
        }
    });
}

public void onActivityResult(int requestCode,int resultCode,Intent intentData){
    super.onActivityResult(requestCode, resultCode, intentData);
    if(requestCode==1 && resultCode==RESULT_OK && intentData!=null){
        ImageView img=(ImageView)findViewById(R.id.imageView1 );

        Bitmap bmp=Bitmap.createBitmap(img.getHeight(),img.getWidth(),Bitmap.Config.RGB_565);
        Canvas cnvs=new Canvas(bmp);
        //img.setImageBitmap(bmp);

        Paint paint=new Paint();
        paint.setColor(Color.RED);

        Uri data=intentData.getData();
        String[] filePath={MediaStore.Images.Media.DATA};
        Cursor cur=getContentResolver().query(data,filePath,null,null,null );
        cur.moveToFirst();
        int colIndex=cur.getColumnIndex(filePath[0]);
        String picPath=cur.getString(colIndex);
        cur.close();
        cnvs.drawRect(20, 20,50,50 , paint);


        img.setImageBitmap(bmp);
        img.setImageBitmap(BitmapFactory.decodeFile(picPath));
    }   
}



        return super.onOptionsItemSelected(item);
}

}