When a timer tick occurs, you want to repaint your PictureBox. To do this, you should make it so that the PictureBox receives a Paint event. This is what Refresh() does, so the call to this.pictureBox1.Refresh(); goes in the end of timer1_Tick.
It doesn't make sense for the Paint event to contain a call to Refresh, because this, in turn, generates a Paint event.