真有意思[ZU14.CN]

/// <summary>
/// 会产生graphics异常的PixelFormat
/// </summary>
private static PixelFormat[] indexedPixelFormats = { PixelFormat.Undefined, PixelFormat.DontCare,
PixelFormat.Format16bppArgb1555, PixelFormat.Format1bppIndexed, PixelFormat.Format4bppIndexed,
PixelFormat.Format8bppIndexed
    };

/// <summary>
/// 判断图片的PixelFormat 是否在 引发异常的 PixelFormat 之中
/// </summary>
/// <param name="imgPixelFormat">原图片的PixelFormat</param>
/// <returns></returns>
private static bool IsPixelFormatIndexed(PixelFormat imgPixelFormat)
{
    foreach (PixelFormat pf in indexedPixelFormats)
    {
        if (pf.Equals(imgPixelFormat)) return true;
    }

    return false;
}

//.........使用
using (Image img = Image.FromFile("原图片路径"))
{
    //如果原图片是索引像素格式之列的，则需要转换
    if (IsPixelFormatIndexed(img.PixelFormat))
    {
        Bitmap bmp = new Bitmap(img.Width, img.Height, PixelFormat.Format32bppArgb);
        using (Graphics g = Graphics.FromImage(bmp))
        {
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
            g.DrawImage(img, 0, 0);
        }
        //下面的水印操作，就直接对 bmp 进行了
        //......
    }
    else //否则直接操作
    {
         //直接对img进行水印操作
    }
}