Hi Terence,
Unfortunately, with the way OAuth Echo works, we can only access the URL that you sign for (in this case, https://api.twitter.com/1/account/verify_credentials.json). This means, that we are unable to post a tweet on your behalf to twitter, because the oauth header you are signing is not for statuses/update.
Twitter is working on a solution for us, but, for the time being, you'll have to post the tweet yourself. When you upload an image, the twitpic url is included in the JSON or XML response, so you can pull the URL from there to insert into your tweet.