Repeat images Android GridView

Im having a little problem with gridview the iamges shows normally but when i scroll down then i scroll up the images positions change or repeat i dont know why iam little bit confused, i use over 50 to 100 images but now only 19 images i was testing how it will be so i got that problem and it wont stick on its position . Thanks .

public class ImageAdapter extends BaseAdapter {
private Context context;

public Integer[] images = {
        R.drawable.f_alicate,R.drawable.f_arcoajustable,
        R.drawable.f_bateria,R.drawable.f_bisagra,
        R.drawable.f_bisagra2,R.drawable.f_cadena_galvanizada,
        R.drawable.f_canilla_bronze ,R.drawable.f_canilla_con_palanca,
        R.drawable.f_casco_rojo,R.drawable.f_casco_rojo2,
        R.drawable.f_cerraduracajon,R.drawable.f_cerradura_para_cajon2,
        R.drawable.f_conexion_cromado,R.drawable.f_cotra_candena,
        R.drawable.f_cuchara_albanil,R.drawable.f_cutter,
        R.drawable.f_cutter2,R.drawable.f_disco,
        R.drawable.f_corta_hiero};

// Constructor
public ImageAdapter(Context c){
    context = c;
}

public int getCount() {
    return images.length;
}

public Object getItem(int position) {
    return images[position];
}

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

public View getView(int position, View convertView, ViewGroup parent) {

    ImageView imageView = null ;

        if (convertView == null) {
            imageView = new ImageView(context);
            new BitmapWorkerTask(imageView).execute(images[position]);

            //create new ImageView if it is not present and populate it with some image
        } else {
            imageView = (ImageView) convertView;
            //re-use ImageView that already exists in memory
        }
    return imageView;
}

class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    private final WeakReference<ImageView> imageViewReference;
    private int data = 0;

    public BitmapWorkerTask(ImageView imageView) {
        // Use a WeakReference to ensure the ImageView can be garbage collected
        imageViewReference = new WeakReference<ImageView>(imageView);
    }

    // Decode image in background.
    @Override
    protected Bitmap doInBackground(Integer... params) {
        data = params[0];
        return decodeSampledBitmapFromResource(ImageAdapter.this.context.getResources(), data, 450, 450);
    }

    // Once complete, see if ImageView is still around and set bitmap.
    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (imageViewReference != null && bitmap != null) {
            final ImageView imageView = imageViewReference.get();
            if (imageView != null) {
                imageView.setImageBitmap(bitmap);
                imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
                imageView.setLayoutParams(new GridView.LayoutParams(450, 450));
            }
        }
    }
}

public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,
                                                     int reqWidth, int reqHeight) {

    // First decode with inJustDecodeBounds=true to check dimensions
    final BitmapFactory.Options options = new BitmapFactory.Options();
    options.inJustDecodeBounds = true;
    BitmapFactory.decodeResource(res, resId, options);

    // Calculate inSampleSize
    options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

    // Decode bitmap with inSampleSize set
    options.inJustDecodeBounds = false;
    return BitmapFactory.decodeResource(res, resId, options);
}

public static int calculateInSampleSize(
        BitmapFactory.Options options, int reqWidth, int reqHeight) {
    // Raw height and width of image
    final int height = options.outHeight;
    final int width = options.outWidth;
    int inSampleSize = 1;

    if (height > reqHeight || width > reqWidth) {

        // Calculate ratios of height and width to requested height and width
        final int heightRatio = Math.round((float) height / (float) reqHeight);
        final int widthRatio = Math.round((float) width / (float) reqWidth);

        // Choose the smallest ratio as inSampleSize value, this will guarantee
        // a final image with both dimensions larger than or equal to the
        // requested height and width.
        inSampleSize = heightRatio < widthRatio ? heightRatio : widthRatio;
    }

    return inSampleSize;
}

In getView(), when re-use ImageView that already exists you still need to set the image of that view. What you do actually is to set the image when it is a new ImageView only.