WebsiteThumbImage.cs

using System;
using System.Drawing;
using System.Text.RegularExpressions;
using System.Threading;
using System.Windows.Forms;

public class WebsiteThumbImage
{
    public int ThumbWidth { get; set; }
    public int ThumbHeight { get; set; }
    public int BrowserWidth { get; set; }
    public int BrowserHeight { get; set; }

    public string Url { get; set; }
    public Bitmap ThumbImage { get; set; }

    public Bitmap Generate()
    {
        if (ValidationControl())
            return null;

        Thread th = new Thread(new ThreadStart(_Execute));
        th.SetApartmentState(ApartmentState.STA);
        th.Start();
        th.Join();

        return ThumbImage;
    }

    // just operational codes :D
    public void _Execute()
    {            
        WebBrowser browser = new WebBrowser();
        browser.ScrollBarsEnabled = false;
        browser.Navigate(this.Url);
        browser.DocumentCompleted += browser_DocumentCompleted;

        while (browser.ReadyState != WebBrowserReadyState.Complete)
            Application.DoEvents();
        browser.Dispose();

    }

    void browser_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
    {
        WebBrowser WebBrowser = (WebBrowser)sender;
        WebBrowser.ClientSize = new Size(this.BrowserWidth, this.BrowserHeight);
        WebBrowser.ScrollBarsEnabled = false;

        this.ThumbImage = new Bitmap(WebBrowser.Bounds.Width, WebBrowser.Bounds.Height);
        WebBrowser.BringToFront();
        WebBrowser.DrawToBitmap(this.ThumbImage, WebBrowser.Bounds);
        this.ThumbImage = (Bitmap)this.ThumbImage.GetThumbnailImage(this.ThumbWidth, this.ThumbHeight, null, IntPtr.Zero);
    }

    private bool ValidationControl() 
    {
        if (String.IsNullOrEmpty(this.Url))
            throw new ArgumentNullException();

        // for url validation
        if (!Uri.IsWellFormedUriString(this.Url, UriKind.RelativeOrAbsolute))
            throw new Exception();

        // another validation method -- using optional
        if (!IsValidUrl(this.Url))
            throw new Exception();

        return true;
    }

    private bool IsValidUrl(string url)
    {
        return Regex.IsMatch(url, @"(http|https)://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?");
    }
}

// Let's use
 string url = ((string.IsNullOrEmpty(Request.Params["site"])) ? "blog.yemrekeskin.com" : Request.Params["site"]);
 int width = ((string.IsNullOrEmpty(Request.Params["width"])) ? 1000 : int.Parse(Request.Params["width"]));
 int height = ((string.IsNullOrEmpty(Request.Params["height"])) ? 940 : int.Parse(Request.Params["height"]));
 int capWidth = ((string.IsNullOrEmpty(Request.Params["capWidth"])) ? 900 : int.Parse(Request.Params["capWidth"]));
 int capHeight = ((string.IsNullOrEmpty(Request.Params["capHeight"])) ? 800 : int.Parse(Request.Params["capHeight"]));

 string address = "http://" + url;
 Bitmap thumbnail = WebsiteThumbnailImageGenerator.GetWebSiteThumbnail(address, capWidth, capHeight, width, height);

 Response.ContentType = "image/jpeg";
 thumbnail.Save(Response.OutputStream, ImageFormat.Jpeg);