using System;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        string json = @"

            {

              ""ImageMap"": {

                ""20896"": [

                  {

                    ""Item1"": ""ImageThumbnail_100_By_44"",

                    ""Item2"": ""VGhlIHF1aWNrIGJyb3duIGZveCBqdW1wZWQgb3ZlciB0aGUgbGF6eSBkb2dzLiBQYWNrIG15IGJveCB3aXRoIGZpdmUgZG96ZW4gbGlxdW9yIGp1Z3Mu""

                  }

                ]

              }

            }";

        Root root = JsonConvert.DeserializeObject<Root>(json);

        Console.WriteLine(root.ImageMap[20896L].First().Item1);

    }

}

​

class Root

{

    [JsonProperty("imageMap")]

    public IDictionary<long, ICollection<Tuple<string, byte[]>>> ImageMap { get; set; }

}

​