public Bitmap GenerateScreenshot(string url)
{
    // This method gets a screenshot of the webpage
    // rendered at its full size (height and width)
    return GenerateScreenshot(url, -1, -1);
}

public Bitmap GenerateScreenshot(string url, int width, int height)
{
    // Load the webpage into a WebBrowser control
    WebBrowser wb = new WebBrowser();
    wb.ScrollBarsEnabled = false;
    wb.ScriptErrorsSuppressed = true;
    wb.Navigate(url);
    while (wb.ReadyState != WebBrowserReadyState.Complete) { Application.DoEvents(); }


    // Set the size of the WebBrowser control
    wb.Width = width;
    wb.Height = height;

    if (width == -1)
    {
        // Take Screenshot of the web pages full width
        wb.Width = wb.Document.Body.ScrollRectangle.Width;
    }

    if (height == -1)
    {
        // Take Screenshot of the web pages full height
        wb.Height = wb.Document.Body.ScrollRectangle.Height;
    }

    // Get a Bitmap representation of the webpage as it's rendered in the WebBrowser control
    Bitmap bitmap = new Bitmap(wb.Width, wb.Height);
    wb.DrawToBitmap(bitmap, new Rectangle(0, 0, wb.Width, wb.Height));
    wb.Dispose();

    return bitmap;
}

// Generate thumbnail of a webpage at 1024x768 resolution
Bitmap thumbnail = GenerateScreenshot("http://pietschsoft.com", 1024, 768);

// Generate thumbnail of a webpage at the webpage's full size (height and width)
thumbnail = GenerateScreenshot("http://pietschsoft.com");

// Display Thumbnail in PictureBox control
pictureBox1.Image = thumbnail;

/*
// Write Image to the Page.Response.OutputStream
Response.ContentType = "image/png";
thumbnail.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Png);

// Save Thumbnail to a File
thumbnail.Save("thumbnail.png", System.Drawing.Imaging.ImageFormat.Png);
*/