I'll start off by saying I'm not quite sure why you need it, given that most Twitter clients allow you to upload photos from within the app, but Yfrog today released their free application to do just that. The app lets you post your photos and videos directly to Twitter, and you can actually view tweets, mentions and messages as well. In a sense you could call it the "Yfrog Twitter Client", but the features are slimmed down enough that it doesn't quite make the cut. If you upload a ton of photos or videos on the go I can see this coming in handy, but for most a reliable Twitter client will get the job done.