Drawing on the screen: Having trouble with borderless and transparent form

I want to make a drawing application that runs on top of the screen, providing a layer of a transparent canvas on top of the screen that can be painted. So it's like drawing directly on the screen when in fact it is not.

I've made ​​a drawing application, and it works very well. Using a borderless form so as to cover all areas of the screen. Using mouse events (MOUSE_DOWN, MOUSE_MOVE, MOUSE_UP) for the drawing process. And Using the PictureBox (pbxCanvas) as Canvas.

But when I change TransparancyKey of the form to white, the same color as the background canvas (which is actually a  general color in bitmap bCanvas), Mouse clicks become pass through form and I can not draw on the canvas.

    TransparencyKey = Color.White;

is there any way that I can achieve this? use mouse events to draw on a transparent canvas, and can save the image created in a bitmap file, without making significant changes in a drawing application that I have created.

**Note**

This event is a mouse that I use to draw:

#region Draw to pbxCanvas
        public void Draw(bool draw, Point sp, Point ep)
        {
            if (draw)
            {
                gCanvas.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
                pen = new Pen(new SolidBrush(ColorName), BrushSize);
                if (toolPen.Checked)
                {
                    gCanvas.DrawLine(pen, sp, ep);
                }
                else if (toolEreser.Checked)
                {
                    Rectangle rect = new Rectangle(ep.X, ep.Y, BrushSize*5, BrushSize*5);
                    gCanvas.DrawEllipse(pen, rect);
                    gCanvas.FillEllipse(new SolidBrush(ColorName), rect);
                }
                pbxCanvas.Refresh();
                dirty = true;
            }
        }

        private void pbxCanvas_MouseDown(object sender, MouseEventArgs e)
        {
            sp = e.Location;
            if (e.Button == MouseButtons.Left)
            {
                ActivePaint = true;
            }
        }

        private void pbxCanvas_MouseUp(object sender, MouseEventArgs e)
        {
            ActivePaint = false;
        }

        private void pbxCanvas_MouseMove(object sender, MouseEventArgs e)
        {
            ep = e.Location;
            Draw(ActivePaint, sp, ep);
            sp = ep;
        }


        #endregion

***Note***: This is WinForm Application.