主要原因就是上传的图片原类型是GIF型就不行，所以要先转位图,将原图像画到位图上,再保存位图
/// <summary>
/// 给图片加水印
/// </summary>
/// <param name="strPath">原始文件</param>
/// <param name="strPathSyp">水印文件</param>
/// <param name="strPathSypf">加水印后的结果文件</param>
public static void AddWaterPic(string strPath, string strPathSyp, string strPathSypf)
{
System.Drawing.Image image = System.Drawing.Image.FromFile(strPath);
Bitmap bm = new Bitmap(image);
System.Drawing.Image copyImage = System.Drawing.Image.FromFile(strPathSyp);
System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bm);
//如果要使水印居中显示，只要更改bm.Width - copyImage.Width, bm.Height - copyImage.Height，这两个值
//就可以了，在实际项目中，我用的是Convert.ToInt32(image.Width * 0.3), Convert.ToInt32(image.Height * 0.3)
g.DrawImage(copyImage, new System.Drawing.Rectangle(bm.Width - copyImage.Width, bm.Height - copyImage.Height, copyImage.Width, copyImage.Height), 0, 0, copyImage.Width, copyImage.Height, System.Drawing.GraphicsUnit.Pixel);
g.Dispose();
copyImage.Dispose();
bm.Save(strPathSypf);
bm.Dispose();
}