I've been on Twitter almost two years and haven't ever seen a picture posted directly in a tweet or in my Twitter stream. Where/How would it post? There isn't enough space and the boxes only allow for characters/text not for files which is what an image would be.
If you found a third party app that does this, please do post back and share what it is. I've used
http://twitwall.com/ that can be used as a sort of Twitter blog equivalent where you create "posts" that will go out on your Twitter account and works like a blog sort of where you can have your posts kept historically. You can text, images, and even video all to your Twitwall post and then it will tweet the post for you. You can also set it to re-tweet the post a couple of times.