I can't speak to ratio of copyrighted to public domain photos on VOA. However, I think we should keep in mind that uploading files from a URL (the upload_by_url user right) is only available to Extended Uploaders, GWToolset users, Admins, and Image Reviewers; users who are already trusted to be familiar with Commons' copyright policy. Anyone who abuses their rights by uploading copyrighted images can have those rights revoked.