There are libraries available to help parse RSS feeds - e.g.
If you want to do a quick "hacky" parse of the string yourself, then you could use RegularExpressions - e.g. a Regex something like thumbnail url='(?<imageurl>http.*?)' height='240' width='226' would pull out your url as the imageurl group
Match match = Regex.Match(input, @"thumbnail url='(?<imageurl>http.*?)' height='240' width='226'");if (match.Success){ string url = match.Groups[1].Value;}