image transperent in picture box

using (Image img = Image.FromFile(filename))
using (Bitmap bmp = new Bitmap(img))
{
    for (int x = 0; x < img.Width; x++)
    {
        for (int y = 0; y < img.Height; y++)
        {
            Color c = bmp.GetPixel(x, y);
            if (c.R == 255 && c.G == 255 && c.B == 255)
                bmp.SetPixel(x, y, Color.FromArgb(0));
        }
    }
    bmp.Save("out.png", ImageFormat.Png);
}