Set copy the left pixel and make any instances of that color transparent

transparentize.cs

ExtendedImage _img = sender as ExtendedImage;

byte transpCorner = _img.Pixels[0 * _img.PixelWidth + 0]; // top left pixel
byte[] newPixels = _img.Pixels;
for (int xp = 0; xp < _img.PixelWidth; xp++)
{
    for (int yp = 0; yp < _img.PixelHeight; yp++)
    {
        int r = 0, g = 0, b = 0, alpha = 0;

        int idx = xp * _img.PixelWidth + yp; 
        byte r1 = (byte)(r * (alpha / 255d));
        byte g1 = (byte)(g * (alpha / 255d));
        byte b1 = (byte)(b * (alpha / 255d));

        if (newPixels[idx] == transpCorner) // if the current pixel is the same as the transpCorner value then set it's alpha to 0
        {
            newPixels[idx] = (byte)((alpha << 24) | (r1 << 16) | (g1 << 8) | b1);
        }
    }
}