collision detection in c#

in panel1; an arrayList or some object to keep track of the image and it's position. if the user drop an
image on top of another image, the is a messegeBox that shows."there is a preexisting image in the
desire location, do you wich to delete the preexisting image; Yes or No". if yes, the old image is deleted
and the new image is place at that location. if No, the old image remains. you can't put a pictureBox on
the top of another in the drop area.

this the program;  you insert pictureBoxs in panel2 and load them with image. you can drag and drop the
image in panel1

bool pbDown = false;
        PictureBox selectedPB;
        Rectangle prevClip;
        PictureBox pb;
        int startX, startY;
        ContextMenu cm = new ContextMenu();

private void DesignForm_Load(object sender, EventArgs e)
        {
            this.Reset();
            prevClip = Cursor.Clip;
            foreach (Control ctl in panel2.Controls)
            {
                if (ctl is PictureBox)
                {
                    ctl.MouseDown += new MouseEventHandler(ctl_MouseDown);
                    ctl.MouseMove += new MouseEventHandler(ctl_MouseMove);
                    ctl.MouseUp += new MouseEventHandler(ctl_MouseUp);
                }
            }
            MenuItem delete = new MenuItem("Delete");
            delete.Click += new EventHandler(delete_Click);
            cm.MenuItems.Add(delete);
            panel1.Click += new EventHandler(panel1_Click);
            panel1.MouseMove += new MouseEventHandler(panel1_MouseMove);

        }

        private void panel1_Click(object sender, EventArgs e)
        {
            if (selectedPB != null)
            {
                selectedPB.BorderStyle = BorderStyle.None;
                selectedPB = null;
            }

        }

        private void panel1_MouseMove(object sender, MouseEventArgs e)
        {
            if (selectedPB != null && pbDown)
            {
                System.Diagnostics.Debug.Print("Too Fast!...panel1_MouseMove(" + e.X.ToString() + ", " + e.Y.ToString() + ")");
                Point pt = panel1.PointToScreen(new Point(e.X, e.Y));
                pt = selectedPB.PointToClient(pt);
                MouseEventArgs mea = new MouseEventArgs(MouseButtons.Left, 0, pt.X, pt.Y, 0);
                pb_MouseMove(selectedPB, mea);
            }
        }
       

        void delete_Click(object sender, EventArgs e)
        {
            MenuItem mi = (MenuItem)sender;
            ContextMenu cm = (ContextMenu)mi.GetContextMenu();
            PictureBox pb = (PictureBox)cm.SourceControl;
            pb.Dispose();
        }

        void ctl_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                Cursor.Clip = this.RectangleToScreen(this.ClientRectangle);
                startX = e.X;
                startY = e.Y;
                PictureBox source = (PictureBox)sender;
                pb = new PictureBox();
                pb.Size = source.Size;
                pb.Location = this.PointToClient(panel2.PointToScreen(source.Location));
                pb.Image = source.Image;
                this.Controls.Add(pb);
                pb.BringToFront();
            }
        }

        void ctl_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                if (pb != null)
                {
                    pb.Location = new Point(pb.Location.X + (e.X - startX), pb.Location.Y + (e.Y - startY));
                    startX = e.X;
                    startY = e.Y;
                }
            }
        }

        void ctl_MouseUp(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                if (pb != null)
                {
                    Cursor.Clip = prevClip;
                    if (pb.ClientRectangle.IntersectsWith(panel1.ClientRectangle))
                    {
                        pb.Hide();
                        pb.Location = panel1.PointToClient(this.PointToScreen(pb.Location));
                        panel1.Controls.Add(pb);
                        pb.ContextMenu = cm;
                        pb.Show();

                        pb.MouseDown += new MouseEventHandler(pb_MouseDown);
                        pb.MouseMove += new MouseEventHandler(pb_MouseMove);
                        pb.MouseUp += new MouseEventHandler(pb_MouseUp);
                    }
                    else
                    {
                        pb.Dispose();
                    }
                    pb = null;
                }
            }
        }

        void pb_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                pbDown = true;
                if (selectedPB != null)
                {
                    selectedPB.BorderStyle = BorderStyle.None;
                    selectedPB = null;
                }
                selectedPB = (PictureBox)sender;
                selectedPB.BorderStyle = BorderStyle.FixedSingle;
                selectedPB.BringToFront();
                Rectangle pnl = this.RectangleToScreen(new Rectangle(panel1.Location, panel1.Size));
                pnl.X = pnl.X + 1;
                pnl.Width = pnl.Width - 2;
                pnl.Y = pnl.Y + 1;
                pnl.Height = pnl.Height - 2;
                Cursor.Clip = pnl;
                startX = e.X;
                startY = e.Y;
            }
        }

        void pb_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                PictureBox localPB = (PictureBox)sender;
                if (localPB == selectedPB)
                {
                    localPB.Location = new Point(localPB.Location.X + (e.X - startX), localPB.Location.Y + (e.Y - startY));
                }
            }
        }

        void pb_MouseUp(object sender, MouseEventArgs e)
        {
            pbDown = false;
            Cursor.Clip = prevClip;
        }

       
       
    }
}