style="android:galleryItemBackground"

<Gallery style="android:galleryItemBackground"
    android:id="@+id/gallery"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_gravity="top" android:layout_marginTop="50dip"/>

gallery.setUnselectedAlpha(0.3f);

public View getView(int position, View convertView, ViewGroup parent) {
    ImageView i = new ImageView(mContext);

    position= getPosition(position);

    i.setImageBitmap(images.elementAt(position));
    i.setScaleType(ImageView.ScaleType.FIT_CENTER);

    i.setBackgroundColor(R.color.chatblue); //add some background, and it work!


    return i;
}