Will change the overall transparency of an image.

TuneTransparency.cs

using System.Drawing;

public static class GraphicHelpers
{
    /// <summary>
    /// Changes the overall transparency of an image
    /// </summary>
    /// <param name="originalImage">The input image to be tuned</param>
    /// <param name="alpha">the desired alpha value. It is based on the 255 alpha pixels</param>
    /// <param name="antialias">To conserve the antialiasing on the image</param>
    /// <returns></returns>
    public static Image TuneTransparency(Image originalImage, int alpha, bool antialias)
    {
        //Cast & clone image to bitmap
        Bitmap bitmap = new Bitmap(originalImage);
        //Cycle through all pixels
        Color pixel;

        //ratio between a max byte and desired alpha byte, this achieves the conservation of antialiasing
        int ratio = 255 / alpha;
        for (int i = 0; i < originalImage.Width; i++)
        {
            for (int j = 0; j < originalImage.Height; j++)
            {
                //Get pixel color
                pixel = bitmap.GetPixel(i, j);

                //If pixel is not already transparent
                if (pixel.A != 0)
                {

                    //Set the pixel to
                    bitmap.SetPixel(i, j, Color.FromArgb(
                        //If antialiasing is enabled the: pixel alpha/ratio else just put the alpha
                        antialias ? pixel.A / ratio : alpha,
                        pixel));
                }
            }
        }
        return bitmap;
    }
}