How about this. You load the html into a webbrowser control and then use the DrawToBitmap method. It doesn't show up on intellisense and this is probably not the best solution, but it works. Observe the DocumentCompleted event and add the following code:
private void webBrowser_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
{
var bmp = new Bitmap(100, 100);
var rect = new Rectangle(webBrowser.Location.X, webBrowser.Location.Y, webBrowser.Width, webBrowser.Height);
webBrowser.DrawToBitmap(bmp, rect);
bmp.Save("test.jpg", ImageFormat.Jpeg);
}
You'll probably want to change the width and height of that bitmap object (do it in some smart way or something). Hope this helps.
EDIT: I see now that you are using a webservice for this, hence this solution probably won't work. I'll leave it here just for information's sake.