var xmlDoc = new XmlDocument();
xmlDoc.Load(@"C:\Users\*\Desktop\*.xml");
var node = xmlDoc.SelectNodes("root/block")[].ChildNodes[];
var cdata = node.FirstChild as XmlCDataSection;
var strImage = cdata.Value;
var ms = new MemoryStream(System.Convert.FromBase64String(strImage));
Image returnImage = Image.FromStream(ms);
pictureBox1.Image = returnImage;
Image myImage = xxxxx;
System.IO.MemoryStream ms = new System.IO.MemoryStream();
myImage.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
result = System.Convert.ToBase64String(ms.ToArray());
var itemNode = xmlDoc.CreateNode(XmlNodeType.Element, "MyImage", null);
itemNode.AppendChild(xmlDoc.CreateCDataSection(result));