逐个像素进行Alpha值的设置，网上其他的代码不能处理有透明背景的图片，因此要对Alpha、R、G、B均为0的透明色进行特殊处理，不做转换。

        private Bitmap SetImageOpacity(Image srcImage, int opacity)
        {
            Bitmap pic = new Bitmap(srcImage);
            for (int w = 0; w < pic.Width; w++)
            {
                for (int h = 0; h < pic.Height; h++)
                {
                    Color c = pic.GetPixel(w, h);
                    Color newC;
                    if (!c.Equals(Color.FromArgb(0, 0, 0, 0)))
                    {
                        newC = Color.FromArgb(opacity, c);
                    }
                    else
                    {
                        newC = c;
                    }
                    pic.SetPixel(w, h, newC);
                }
            }
            return pic;
        }

        private Image SetImageOpacity2(Image srcImage, int opacity)
        {
            Bitmap img = new Bitmap(srcImage);
            using (Bitmap bmp = new Bitmap(img.Width, img.Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb))
            {
                using (Graphics g = Graphics.FromImage(bmp))
                {
                    g.DrawImage(img, 0, 0);
                    for (int h = 0; h <= img.Height - 1; h++)
                    {
                        for (int w = 0; w <= img.Width - 1; w++)
                        {
                            Color c = img.GetPixel(w, h);
                            if (!c.Equals(Color.FromArgb(0, 0, 0, 0)))
                            {
                                bmp.SetPixel(w, h, Color.FromArgb(opacity, c.R, c.G, c.B));
                            }
                            else
                            {
                                bmp.SetPixel(w, h, Color.FromArgb(c.A, c.R, c.G, c.B));
                            }
                        }
                    }
                }
                return (Image)bmp.Clone();
            }
        }