Twitter added its own photo upload feature, after years of clients adding photos via APIs such as the TwitPic API. The new feature will be slowly rolled out on Twitter.com to all users. It is also expected to be incorporated into Twitter's official apps, as well as the Twitter API, upon which Twitter builds all of its end-user services.
A developer discussion thread from Twitter's Arnaud Meunier explains the new feature:
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.
Though the writeable portion of the photo API is not yet available, Twitter has already added a photo entity for including Twitter photos in your applications. Much like location added to tweets, images are now considered meta-data, not just a URL tacked on to a tweet.
The API includes multiple sizes for every image, so that applications can retrieve the appropriate size for a specific context. For example, the Twitter for iPhone client already shows thumbnail previews of images. Now the API will be able to support that functionality. There's also a field called "type" that currently only supports photos. One can extrapolate that other media may be coming next, including video, especially as more people are recording and sharing via mobile.