using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var s = @"this is my text here is a link http://d...content-available-to-author-only...f.com and my is /images/dd.gif";
        var regex = new Regex(@"\S*/images\S*|(?<!\S)((?:(?:https?|ftp)://)?[\w-]+(?:(?:\.[\w-]+)+)(?:[\w.,@?^=%&:/~+#-]*[\w@?^=%&/~+#-])?)");
        var result = regex.Replace(s, match => match.Groups[1].Success ? "" : match.Value);
        Console.WriteLine(result);
	}
}