Twitter and Twitpic are two different sites. So you actually have to tweet the link of the photo you want people to see on twitter. This is usually done automatically depending on what client you’re are using for Twitter, e.g. HooteSuite, Tweetdeck.

If you want to sent a public message to someone specifically, you would just add the @username of the person you want to talk to. I have detailed information about how this all works in my free Beginner’s Guide to Twitter. If you haven’t already, check it out here: http://DaveCharest.com/twitter-guide-1

Hope this helps. Thanks for the comment!