Change Bitmap Opacity

using System;
using System.Drawing;
using System.Drawing.Imaging;

namespace ImageUtils
{
    class ImageTransparency
    {
        public static Bitmap ChangeOpacity(Image img, float opacityvalue)
        {
            // Create a new bitmap with same dimensions as original
            Bitmap bmp = new Bitmap(img.Width,img.Height);
            ColorMatrix colormatrix = new ColorMatrix();
            colormatrix.Matrix33 = opacityvalue;
            ImageAttributes imgAttribute = new ImageAttributes();
            imgAttribute.SetColorMatrix(colormatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
            Rectangle srcRect = new Rectangle(0, 0, bmp.Width, bmp.Height);
            using (Graphics graphics = Graphics.FromImage(bmp))
            {
                graphics.DrawImage(img, srcRect, 0, 0, img.Width, img.Height,
                    GraphicsUnit.Pixel, imgAttribute);
            }
            return bmp;
        }
    }
}