ImageView activityImageViews[] = {
    (ImageView) rootView.findViewById(R.id.img_activity_1),
    (ImageView) rootView.findViewById(R.id.img_activity_2),
    (ImageView) rootView.findViewById(R.id.img_activity_3),
    (ImageView) rootView.findViewById(R.id.img_activity_4)
};

// in the header:
@InjectView(R.id.imageview1) ImageView imageView1;
@InjectView(R.id.imageview2) ImageView imageView2;
@InjectView(R.id.imageview3) ImageView imageView3;

// inside your code:
ImageView activityImageViews[] = {
    imageView1, imageView2, imageView3
};

ImageView activityImageViews[] = {
    ButterKnife.findById(view, R.id.img_activity_1),
    ButterKnife.findById(view, R.id.img_activity_2),
    ButterKnife.findById(view, R.id.img_activity_3),
    ButterKnife.findById(view, R.id.img_activity_4)
};