paint问题（急）好心人帮帮忙

在网上找了一段代码，主要是实现picturebox选中效果。但是当pictruebox处于选中状态时，只要往里面放带边框的窗体，就会出项边框。
收回下拉列表的时候就变这样了：
各位大神帮忙啊。    PictureBox old = null;
        private void PictureBox_MouseDown(System.Object sender, System.Windows.Forms.MouseEventArgs e)
        {
            if (e.Button == System.Windows.Forms.MouseButtons.Left)
            {
                PictureBox pic = (PictureBox)(sender);
                if (pic == old) return;
                if (old != null)
                {
                    old.Width -= 10;
                    old.Height -= 10;
                    old.Location = new Point(old.Location.X + 5, old.Location.Y + 5);
                }
                old = pic;
                pic.Width += 10;
                pic.Height += 10;
                pic.Location = new Point(pic.Location.X - 5, pic.Location.Y - 5);
                pic.BringToFront();
             
            }
        }
        private void pictureBox_Paint(object sender, PaintEventArgs e)
        {
            PictureBox p = (PictureBox)sender;
            if (p == old)
            {
                Pen pp = new Pen(Color.Yellow);
                pp.Width = 5;
                e.Graphics.DrawRectangle(pp, e.ClipRectangle.X, e.ClipRectangle.Y, e.ClipRectangle.X + e.ClipRectangle.Width - 1, e.ClipRectangle.Y + e.ClipRectangle.Height - 1);
            }
        }