The System.Drawing.Image.FromStream class expects a stream with bytes from an image. You are passing html instead which is not the expected format. To verify this, replace your url to download a specific image like:

 System.Net.WebRequest webRequest = WebRequest.Create("http://localhost:58534/EFTestApp/images/animage.jpg");

Your current code should work. 

  To capture a screen shot of a webpage, you could use the WebBrowser class which has this method DrawToBitmap.

hope it helps