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();}}