I'm not quite sure i understood the followup but just atop of my head, something like this maybe?
string temp = "http://www.mysite.com/funny/vacation/pictures";
string new_word = "really";
string last = temp.Replace("funny", new_word + "/funny");
Or you could use string.Split() or regular expressions or string.Insert(). It would be easier if you have provided a cleaner description of the problem.
Regards,
Anıl Yıldız.