By Alexei Oreskovic SAN FRANCISCO (Reuters) – Twitpic, an early online service that allows Internet users to share photos on the Twitter Inc social network, is shutting down due to what it said were legal threats from Twitter. The dispute revolves around Twitpics effort to have its name approved as a trademark, which Twitter appears to believe violates its own trademark. Twitpic gained a loyal following after its 2008 launch for allowing users to post photos on Twitters social network. Twitter has since added its own photo-sharing feature directly within its Web service.