We have just announced a couple of new exciting features at the All Things D conference, one of which is the ability to upload photos to twitter.com
Uploading photos to Twitter is currently available on the twitter.com
desktop version, and its access is initially limited to a very small number of users. In the next couple of weeks, as we progressively ramp up the number of users who have access to the feature, we'll provide you with more details about how you can use the "Tweet-with-photo" API.
For now, the only change you'll notice on the API side is a new "media" entity in the status object. We encourage you to support rendering, and to help you with that, we just revamped the Tweet Entities documentation page, describing in detail how you can use this structured data: http://dev.twitter.com/pages/tweet_entities
Note that we also updated our API Terms of Service  and our Display Guidelines  to include this new feature.
As ever, if you have questions about the new media entity or our ToS changes, let us know on the list or through @twitterapi.
Arnaud / @rno