Switching Between Different Images Using Seekbar

I am trying to switch between six different images based upon the value of the seekbar (0-5). I am stuck and would welcome any suggestions/help. It compiles and runs, but the app crashes as soon as I try and click on question 1.

Here is what I have for the seekbar:

public void seekbar() {
        seek_bar = (SeekBar) findViewById(R.id.seekBar);
        text_view = (TextView) findViewById(R.id.seekBarText);
        text_view.setText("Stress Level : " + seek_bar.getProgress() + " / " + seek_bar.getMax());

        Intent i = getIntent();
        s = (ArrayList<String>)i.getSerializableExtra("survey");
        String qone = s.get(0);
        //TODO: Set the Default of the Seek Bar

        seek_bar.setOnSeekBarChangeListener(
                new SeekBar.OnSeekBarChangeListener() {

                    int progress_value;

                    @Override
                    public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                        progress_value = progress;
                        text_view.setText("Stress Level : " + progress + " / " + seek_bar.getMax());
                        Toast.makeText(getApplicationContext(), "Changing seekbar's progress", Toast.LENGTH_SHORT).show();

                        if (progress_value == 0) {
                            ImageView photo1 = (ImageView) findViewById(R.id.photo1);
                            int image = R.drawable.placeholder1;
                            photo1.setImageResource(image);
                        }
                        if (progress_value == 1) {
                            ImageView photo2 = (ImageView) findViewById(R.id.photo2);
                            int image = R.drawable.placeholder2;
                            photo2.setImageResource(image);
                        }
                        if (progress_value == 2) {
                            ImageView photo3 = (ImageView) findViewById(R.id.photo3);
                            int image = R.drawable.placeholder3;
                            photo3.setImageResource(image);
                        }
                        if (progress_value == 3) {
                            ImageView photo4 = (ImageView) findViewById(R.id.photo4);
                            int image = R.drawable.placeholder4;
                            photo4.setImageResource(image);
                        }
                        if (progress_value == 4) {
                            ImageView photo5 = (ImageView) findViewById(R.id.photo5);
                            int image = R.drawable.placeholder5;
                            photo5.setImageResource(image);
                        }
                        if (progress_value == 5) {
                            ImageView photo6 = (ImageView) findViewById(R.id.photo6);
                            int image = R.drawable.placeholder6;
                            photo6.setImageResource(image);
                        }

                    }
                        public void onStartTrackingTouch(SeekBar seekbar){
                            Toast.makeText(getApplicationContext(), "Started tracking seekbar", Toast.LENGTH_SHORT).show();

                        }
                        public void onStopTrackingTouch(SeekBar seekbar){
                            text_view.setText("Covered: " + progress_value + "/" + seek_bar.getMax());

                            Toast.makeText(getApplicationContext(), "Stopped tracking seekbar", Toast.LENGTH_SHORT).show();


}


        }
        );
        }