我不熟悉ImageAttributes方法，但是您应该能够简单遍历图像的所有像素并修改像素颜色的alpha分量。

从CodeProject尝试这一操作- 在C＃中更改图像的不透明度 ：

/// <summary>  
/// method for changing the opacity of an image  
/// </summary>  
/// <param name="image">image to set opacity on</param>  
/// <param name="opacity">percentage of opacity</param>  
/// <returns></returns>  
public Image SetImageOpacity(Image image, float opacity)  
{  
    try  
    {  
        //create a Bitmap the size of the image provided  
        Bitmap bmp = new Bitmap(image.Width, image.Height);  

        //create a graphics object from the image  
        using (Graphics gfx = Graphics.FromImage(bmp)) {  

            //create a color matrix object  
            ColorMatrix matrix = new ColorMatrix();      

            //set the opacity  
            matrix.Matrix33 = opacity;  

            //create image attributes  
            ImageAttributes attributes = new ImageAttributes();      

            //set the color(opacity) of the image  
            attributes.SetColorMatrix(matrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);    

            //now draw the image  
            gfx.DrawImage(image, new Rectangle(0, 0, bmp.Width, bmp.Height), 0, 0, image.Width, image.Height, GraphicsUnit.Pixel, attributes);
        }
        return bmp;  
    }  
    catch (Exception ex)  
    { 
        MessageBox.Show(ex.Message);  
        return null;  
    }  
} 

您循环播放像素并仅播放Alpha通道。如果使用Bitmap.LockBits执行此操作，则实际上将非常快。

    private const int bytesPerPixel = 4;

    /// <summary>
    /// Change the opacity of an image
    /// </summary>
    /// <param name="originalImage">The original image</param>
    /// <param name="opacity">Opacity, where 1.0 is no opacity, 0.0 is full transparency</param>
    /// <returns>The changed image</returns>
    public static Image ChangeImageOpacity(Image originalImage, double opacity)
    {
        if ((originalImage.PixelFormat & PixelFormat.Indexed) == PixelFormat.Indexed)
        {
            // Cannot modify an image with indexed colors
            return originalImage;
        }

        Bitmap bmp = (Bitmap)originalImage.Clone();

        // Specify a pixel format.
        PixelFormat pxf = PixelFormat.Format32bppArgb;

        // Lock the bitmap's bits.
        Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
        BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, pxf);

        // Get the address of the first line.
        IntPtr ptr = bmpData.Scan0;

        // Declare an array to hold the bytes of the bitmap.
        // This code is specific to a bitmap with 32 bits per pixels 
        // (32 bits = 4 bytes, 3 for RGB and 1 byte for alpha).
        int numBytes = bmp.Width * bmp.Height * bytesPerPixel;
        byte[] argbValues = new byte[numBytes];

        // Copy the ARGB values into the array.
        System.Runtime.InteropServices.Marshal.Copy(ptr, argbValues, 0, numBytes);

        // Manipulate the bitmap, such as changing the
        // RGB values for all pixels in the the bitmap.
        for (int counter = 0; counter < argbValues.Length; counter += bytesPerPixel)
        {
            // argbValues is in format BGRA (Blue, Green, Red, Alpha)

            // If 100% transparent, skip pixel
            if (argbValues[counter + bytesPerPixel - 1] == 0)
                continue;

            int pos = 0;
            pos++; // B value
            pos++; // G value
            pos++; // R value

            argbValues[counter + pos] = (byte) (argbValues[counter + pos] * opacity);
        }

        // Copy the ARGB values back to the bitmap
        System.Runtime.InteropServices.Marshal.Copy(argbValues, 0, ptr, numBytes);

        // Unlock the bits.
        bmp.UnlockBits(bmpData);

        return bmp;
    }