Example code to follow:

protected void CleanImageFolder()
{
    string imgFolder = Server.MapPath("~/lab/maskemail/img/");
    string[] imgList = Directory.GetFiles(imgFolder, "*.jpg");
    foreach (string img in imgList)
    {
        FileInfo imgInfo = new FileInfo(img);
        if (imgInfo.LastWriteTime < DateTime.Now.AddMinutes(-3))
        {
            imgInfo.Delete();
        }
    }
}