如何改变picturebox的背景

[ssq4706]

private void DTM_ChangePic_Click(object sender, EventArgs e)
        {
            string PicSrc = "";
            openFileDialog1.FileName = "";
            openFileDialog1.Filter = "图片文件(*.jpg)|*.jpg|(*.JPEG)|*.jpeg|(*.PNG)|*.png";
            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                string selectedImg = openFileDialog1.FileName;
                string names = selectedImg.Substring(selectedImg.LastIndexOf("\\") + 1).ToLower();
                Directory.CreateDirectory(@"C:\aa");//创建本地文件夹
                PicSrc = @"C:\aa\" + names;
                File.Copy(selectedImg, PicSrc);//保存选择文件
                pictureBox1.BackgroundImage = Image.FromFile(selectedImg);
                SavaPicSrc(PicSrc);
            }
            
        }

        public void SavaPicSrc(string FileName)
        {
            XmlDocument xmlDoc = new XmlDocument();
            XmlDeclaration del = xmlDoc.CreateXmlDeclaration("1.0", "utf-8", null);
            xmlDoc.AppendChild(del);

            XmlElement root = xmlDoc.CreateElement("Info");//根节点

            XmlElement User = xmlDoc.CreateElement("Pic");
            User.SetAttribute("src", FileName);

            root.AppendChild(User);

            xmlDoc.AppendChild(root);
            xmlDoc.Save("PicSrc.xml");
        }

我这样是更改了背景图片，但是保存不了XML,为什么啊，