You now need not use an external client to share an image on twitter. Twitter toyed with the idea of photo sharing with in its main interface, tested it with a select number of users and now its fully rolled out. You get to see this once you login to twitter and get this highlighted image showing you just how to do it.
Image upload max size will be 3mb and the image will be scaled to fit in your right side panel. You will still be able to use all other image sharing clients, this is just a plus, I think twitter wants image related ads, just like yfrog and twitpic do. More on this and how to do the image sharing here. Tell us by the comments if you are already using the feature, and how you see it.
Ofcourse the tweeps using clients fully will miss noticing this feature for some time. So far I havent seen the feature on the mobile version of twitter on my phone, have you?