I just tried out PhotoNest, a cool new way to view just the images posted to your Twitter stream. It picks up just the photos, and presents them in a slide format, captioned with the tweet they were posted with. You can quickly flick through and see what your friends are up to (based on the photos they post).
Given that Twitter doesn't have a genuine photo-posting mechanism, this seems like a grand idea to me. I've done something similar to collect and expand just the links posted to my stream, but this app does a great job of pulling photos posted with multiple services and presenting them in an easy-to-navigate format.
The only trouble I ran into with PhotoNest was a login issue: when I mistyped my password on the first try, it gave me an error and took me back to the login screen. After that, though, a correct password just kept landing me back at that screen without any message. However, quitting the app and starting it again logged me in automatically without a hitch, so I assume it's a small bug that will be squashed in the next update.
The app will also let you post photos (with a tweet) to your Twitter account, and you can filter your view based on favorite contacts. PhotoNest is available on the App Store for $1.99US. If your Twitter friends post a lot of photos (and people with iPhones tend to), it's a fun way to keep up and worth checking out.