Step 1: Add the below namesapce at the top of the code behind.

 using System.Net;
using System.Text.RegularExpressions;
using System.IO;

Step 2: Add the following method which will upload the file.

 public string UploadFile()
{
try
{
    string filePath = Server.MapPath(@"FolderNamefile.jpg");
    string FtpUrl =  "ftp://ftp.domain.com/";
    string userName = "ftp_user";      
    string password =   "ftp_password";
    string UploadDirectory = "";
    string PureFileName = new FileInfo(filePath).Name;
    String uploadUrl = String.Format("{0}{1}/{2}", FtpUrl, UploadDirectory, PureFileName);
    FtpWebRequest req = (FtpWebRequest)FtpWebRequest.Create(uploadUrl);
    req.Proxy = null;
    req.Method = WebRequestMethods.Ftp.UploadFile;
    req.Credentials = new NetworkCredential(userName, password);
    req.UseBinary = true;
    req.UsePassive = true;
    byte[] data = File.ReadAllBytes(filePath);
    req.ContentLength = data.Length;
    Stream stream = req.GetRequestStream();
    stream.Write(data, 0, data.Length);
    stream.Close();
    FtpWebResponse res = (FtpWebResponse)req.GetResponse();
    return res.StatusDescription;
}
catch (Exception ex)
{
    return "";
}

}