Set image opacity (See related posts)

        public static Image SetOpacity(Image original, float opacity)
        {
            Bitmap temp = new Bitmap(original.Width, original.Height);
            Graphics g = Graphics.FromImage(temp);
            ColorMatrix cm = new ColorMatrix();
            cm.Matrix33 = opacity;

            ImageAttributes ia = new ImageAttributes();
            ia.SetColorMatrix(cm, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
            g.DrawImage(original, new Rectangle(0, 0, temp.Width, temp.Height), 0, 0, original.Width, original.Height, GraphicsUnit.Pixel, ia);
            g.Dispose();

            return temp;
        }