PictureBox Transparency background appears Black after Graphics.Clear(Color.Transparent)

Question

Hello,

I've been struggling with this for quite a while and can't get it fixed. I'm busy with a GIS app. For this I have two PictureBoxes. The first contains all the polygon shapes. the second, I would like to use as a type of overlay over the first, so that I don't need to redraw all the polygons over and over again. When adding the Overlay PictureBox over the First, I can still see the First through the Overlay PictureBox because the Overlay PictureBox has a BackColor of Transparent. But now, the problem: At the moment I have a MouseMove event for the Second PictureBox to draw a rectangular SelectionBox. The code is as follows :

 private void Map_MouseMove(object sender, MouseEventArgs e)
        {
            mouseMoveX = (float)e.X;
            mouseMoveY = (float)e.Y;
            selectBoxGraphics = overlayBox.CreateGraphics();
      
            if (e.Button == MouseButtons.Left)
            {
                selectBoxGraphics.Clear(Color.Transparent);
               Pen myPen = new Pen(Color.Red, 1.5f);
               myPen.DashStyle = System.Drawing.Drawing2D.DashStyle.Dash;
               selectBoxGraphics.DrawRectangle(myPen, GetProperSelectRec());
               myPen.Dispose();
            }
        }

The problem code here is "selectBoxGraphics.Clear(Color.Transparent)". This immediately sets the background of the PictureBox to Black, so I can't see the First PictureBox. Why is this? and How do I fix it?

Any help will be appreciated

Thank you
Nathanael