// @nuget: HtmlAgilityPack

​

using System;

using HtmlAgilityPack;

public class Program

{

    public static void Main()

    {

        var html = 

        @"<div>

            <p>This is some text</p>

            <img src=""http://www.mywebsite.com/myImage1.jpg"" alt = """" style=""width:600px; height: 400px;""/>

            <img src=""http://www.mywebsite.com/myImage2.jpg"" alt = """" style=""width:600px; height: 400px;""/>

            <img src=""http://www.mywebsite.com/myImage3.jpg"" alt = """" style=""width:600px; height: 400px;""/>

        </div>";

​

        var htmlDoc = new HtmlDocument();

        htmlDoc.OptionAutoCloseOnEnd = true;

        htmlDoc.LoadHtml(html);

​

        var htmlNodes = htmlDoc.DocumentNode.SelectNodes("//img");

        foreach (var node in htmlNodes){

            // Adding class "img-responsive"

            node.AddClass("img-responsive");

            // Removing style attribute

            node.Attributes["style"].Remove();

            Console.WriteLine(node.OuterHtml);

        }

        // Adding the close </img> to each images og the HTML

        HtmlNode.ElementsFlags["img"] = HtmlElementFlag.Closed;

        // Here you can see the changes in the HTML string

        Console.WriteLine(htmlDoc.DocumentNode.OuterHtml);

    }

}