You are not copying the bitmap to the clipboard but the PictureBox. Try this code.
Bitmap bmp = new Bitmap(Width, Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
Graphics g = Graphics.FromImage(bmp);
g.FillRectangle(Brushes.White, 0, 0, Width, Height);
PictureBox pb = new PictureBox();
pb.Image = bmp;
Clipboard.SetDataObject(bmp);