Image Drawing

System.Drawing.Image img = System.Drawing.Image.FromFile(Server.MapPath("OriginalImagePath"));
Bitmap bit = new Bitmap(img);

System.Drawing.Image mask = System.Drawing.Image.FromFile(Server.MapPath("MaskImagePath"));
Bitmap bitMask = new Bitmap(mask);

for (int y = 0; y < img.Height; y++)
{
    for (int x = 0; x < img.Width; x++)
    {
       if (bit.GetPixel(x, y).A != 0)
       {
            bit.SetPixel(x, y, bitMask.GetPixel(x, y));
            //bit.SetPixel(x, y, Color.Red);
       }
    }
}