using System;

using System.Text.RegularExpressions;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        Dictionary<String, String> imagePathsKeys = new Dictionary<String, String>();

        var content = "![First Image Description](/image-path.png) ![Second Image Description](/image-path.png)";

        // This regex will only match whatever is between / and .

        // You may have to play around with it so it finds image names more accurately in your actual project

        var matches = new Regex(@"(?<=\/)(.*?)(?=\.)").Matches(content);

        foreach (Match match in matches)

        {

            GroupCollection groups = match.Groups;

            var guid = Guid.NewGuid().ToString(); // generating new guid

            imagePathsKeys.Add(guid, groups[0].Value); // adding new guid as key and image path as value

            content = content.Replace(groups[0].Value, guid); // replacing image path with new guid

        }

        Console.WriteLine(content);

    }

}