Create a sub-class User with an id field and screen_name field, like this:
public class User
{
public string id { get; set; }
public string screen_name { get; set; }
}
public class Response {
public string id { get; set; }
public string text { get; set; }
public string url { get; set; }
public string width { get; set; }
public string height { get; set; }
public string size { get; set; }
public string type { get; set; }
public string timestamp { get; set; }
public User user { get; set; }
}