How to make an image transparent.

Sometimes, we are needed to make an image transparent-.to use it as an watermark or something simillar. We need to use code as following to convert an existing image to an transparent one.

public Bitmap createwatermark(string filepath)
        {
            float[][] matrixItems ={
   new float[] {1, 0, 0, 0, 0},
   new float[] {0, 1, 0, 0, 0},
   new float[] {0, 0, 1, 0, 0},
   new float[] {0, 0, 0, 0.3f, 0}, // the fourth value (0.3f) is the alpha value for partial transparency....
   new float[] {0, 0, 0, 0, 1}};
            ColorMatrix colorMatrix = new ColorMatrix(matrixItems);
            ImageAttributes imageAtt = new ImageAttributes();
            imageAtt.SetColorMatrix(
               colorMatrix,
               ColorMatrixFlag.Default,
               ColorAdjustType.Bitmap);
            Bitmap sourceimage = new Bitmap(filepath);
            Bitmap bitmap = new Bitmap(sourceimage.Width, sourceimage.Height);
            Graphics g = Graphics.FromImage(bitmap);

            g.DrawImage(sourceimage, new Rectangle(0, 0, bitmap.Width, bitmap.Height),0.0f,0.0f,sourceimage.Height,sourceimage.Height,GraphicsUnit.Pixel,imageAtt);
            return bitmap;
        }