I figured out why this was happening.
I didn't realize that [composeViewController addImage:[UIImage imageNamed:@"twitter_logo.png"]];
was technically posting a link to a picture in the tweet, like, http://pic.twitter.com/7Fsa2kadf so that was taking up 32 characters in the tweet.
If I comment out that line of code it is at 140 characters.
Maybe apple will display a link in future updates.