That's because Twitter just launched its own photo service, allowing you to insert pictures in your tweets. The photos will appear as links although the Twitter site and several Twitter clients will let you see the picture right away anyhow. The links appear as pic.twitter.com/(id). Companies like yFrog, TwitPic, and Lockerz have been helping Tweeters post pictures but that business can get kind of skeevy: ownership and rights of the pictures have been at issue and it's just one more third party site that you have to visit and may not trust and there are ads. With this move, and it works pretty well, Twitter further solidifies control over its own ecosystem in a way similar to how it's been driving third party client software companies out of business for a while.