//some previous code
this.pictureBox.Left = left;
this.pictureBox.Top = top;
this.AutoScroll = true;

// add this line under:
this.AutoScrollPosition = new Point(this.Width / 2, this.Height / 2);

public ScalablePictureBox()
{
    //some code ...


    this.pictureTracker.ScrollPictureEvent += new PictureTracker.ScrollPictureEventHandler(this.scalablePictureBoxImp.OnScrollPictureEvent);
    this.pictureTracker.PictureTrackerClosed += new PictureTracker.PictureTrackerClosedHandler(this.pictureTracker_PictureTrackerClosed);

    //Enter the line below to hook on event
     this.scalablePictureBoxImp.ZoomRateChangedEvent += ScalablePictureBoxImp_ZoomRateChangedEvent;
}

private void ScalablePictureBoxImp_ZoomRateChangedEvent(int zoomRate, bool isFullPictureShown)
{
    this.scalablePictureBoxImp.AutoScrollPosition = new Point(this.Width / 2, this.Height / 2);
}