public static final int REQUEST_PICK_IMAGE = 1000;

btnOpenGallery.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
                intent.setType("image/*");
                startActivityForResult(intent, REQUEST_PICK_IMAGE);

            }
        });

@Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {

        super.onActivityResult(requestCode, resultCode, data);

        if(resultCode == RESULT_OK) {
            switch (requestCode) {

                case REQUEST_PICK_IMAGE:
                    imageUri = data.getData();

                    processImage( imageUri  );

                    break;

                default:
                    break;
            }
        }
    }