We recently had a requirement to download an image from a Forms Based SharePoint site programatically but we always got 403 error (Forbidden) when trying to download using WebClient.DownloadData method.
Fortunately, after some googling, we found this great post which helped me in resolving the problem.
On a high level, this approach uses the SharePoint's Authentication.asmx web service to hit the site using the credentials.
Hope this helps. Have a great year ahead....!