using HtmlAgilityPack;

using System;

public class Program

{

    public static void Main()

    {

        HtmlDocument doc = new HtmlDocument();

        var html = "<img src=\"https://thumbs.com/thumbs/test.mp4/test1.mp4-3.jpg\" alt=\"This is the description i want to get too\" >";

        doc.LoadHtml(html);

        HtmlNode image = doc.DocumentNode.SelectSingleNode("//img");

​

        Console.WriteLine("Source: {0}", image.Attributes["src"].Value);

        Console.WriteLine("Description: {0}", image.Attributes["alt"].Value);

        Console.Read();

    }

}