For some reason libraries are made a bit different depending on language installed.
Also a signature can hold a logo-image, wich I do not know why, but it is made in 2 files in 2 different sizes.
private string ReadSignature()
    {
        string appDataDir = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData) + "\\Microsoft\\Signatures";
        string signature = string.Empty;
        DirectoryInfo diInfo = new DirectoryInfo(appDataDir);
        if (diInfo.Exists)
        {
            FileInfo[] fiSignature = diInfo.GetFiles("*.htm");
            if (fiSignature.Length > 0)
            {
                StreamReader sr = new StreamReader(fiSignature[0].FullName, Encoding.Default);
                signature = sr.ReadToEnd();
                if (!string.IsNullOrEmpty(signature))
                {
                    string fileName = fiSignature[0].Name.Replace(fiSignature[0].Extension, string.Empty);
                    signature = signature.Replace(fileName + "_files/", appDataDir + "/" + fileName + "_files/");
                }
            }
        }
        else
        {
            appDataDir = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData) + "\\Microsoft\\Signaturer";
            signature = string.Empty;
            diInfo = new DirectoryInfo(appDataDir);
            if (diInfo.Exists)
            {
                FileInfo[] fiSignature = diInfo.GetFiles("*.htm");
                if (fiSignature.Length > 0)
                {
                    StreamReader sr = new StreamReader(fiSignature[0].FullName, Encoding.Default);
                    signature = sr.ReadToEnd();
                    if (!string.IsNullOrEmpty(signature))
                    {
                        string fileName = fiSignature[0].Name.Replace(fiSignature[0].Extension, string.Empty);
                        signature = signature.Replace(fileName + "_files/", appDataDir + "/" + fileName + "_files/");
                    }
                }
            }
        }
        if (signature.Contains("img"))
        {
            int position = signature.LastIndexOf("img");
            int position1 = signature.IndexOf("src", position);
            position1 = position1 + 5;
            position = signature.IndexOf("\"", position1);
            billede1 = appDataDir.ToString() + "\\" + signature.Substring(position1, position - position1);
            position = billede1.IndexOf("/");
            billede1 = billede1.Remove(position, 1);
            billede1 = billede1.Insert(position, "\\");
            billede1 = System.Web.HttpUtility.UrlDecode(billede1);
            position = signature.LastIndexOf("imagedata");
            position1 = signature.IndexOf("src", position);
            position1 = position1 + 5;
            position = signature.IndexOf("\"", position1);
            billede2 = appDataDir.ToString() + "\\" + signature.Substring(position1, position - position1);
            position = billede2.IndexOf("/");
            billede2 = billede2.Remove(position, 1);
            billede2 = billede2.Insert(position, "\\");
            billede2 = System.Web.HttpUtility.UrlDecode(billede2);
        }
        return signature;
    }
Getting and inserting the signature:
Global variables:
    string billede1 = string.Empty;    // holding image1
    string billede2 = string.Empty;    // holding image2
                        string signature = ReadSignature();
                        if (signature.Contains("img"))
                        {
                            int position = signature.LastIndexOf("img");
                            int position1 = signature.IndexOf("src", position);
                            position1 = position1 + 5;
                            position = signature.IndexOf("\"", position1);
                            //CONTENT-ID
                            const string SchemaPR_ATTACH_CONTENT_ID = @"http://schemas.microsoft.com/mapi/proptag/0x3712001E";
                            string contentID = Guid.NewGuid().ToString();
                            //Attach image
                            mailItem.Attachments.Add(@billede1, Microsoft.Office.Interop.Outlook.OlAttachmentType.olByValue, mailItem.Body.Length, Type.Missing);
                            mailItem.Attachments[mailItem.Attachments.Count].PropertyAccessor.SetProperty(SchemaPR_ATTACH_CONTENT_ID, contentID);
                            //Create and add banner
                            string banner = string.Format(@"cid:{0}", contentID);
                            signature = signature.Remove(position1, position - position1);
                            signature = signature.Insert(position1, banner);
                            position = signature.LastIndexOf("imagedata");
                            position1 = signature.IndexOf("src", position);
                            position1 = position1 + 5;
                            position = signature.IndexOf("\"", position1);
                            //CONTENT-ID
                           // const string SchemaPR_ATTACH_CONTENT_ID = @"http://schemas.microsoft.com/mapi/proptag/0x3712001E";
                            contentID = Guid.NewGuid().ToString();
                            //Attach image
                            mailItem.Attachments.Add(@billede2, Microsoft.Office.Interop.Outlook.OlAttachmentType.olByValue, mailItem.Body.Length, Type.Missing);
                            mailItem.Attachments[mailItem.Attachments.Count].PropertyAccessor.SetProperty(SchemaPR_ATTACH_CONTENT_ID, contentID);
                            //Create and add banner
                            banner = string.Format(@"cid:{0}", contentID);
                            signature = signature.Remove(position1, position - position1);
                            signature = signature.Insert(position1, banner);
                        }
                        mailItem.HTMLBody = mailItem.Body + signature;
The stringhandling can be don smarter, but this Works and gave me my sinature
God luck.