The Tumblr app for iOS lets you interact with the social network by posting just about anything from your iPhone, and a recent redesign of the interface makes it even easier.
The Tumblr app interface gives you a Home screen with the latest posts from the people you follow and buttons across the bottom for navigation. When you touch the Tags icon, you get a list of random photos you can swipe through to browse (great for discovery), and you can enter a tag if you want to look at posts with a specific tag. You can touch the profile icon to view your profile information, and see notifications, people you're following, and a discovery tool for finding new blogs. On the bottom right there is a post button: you can choose a post type (from an elegant list), then post to your Tumblr from your iPhone. If you have several Tumblr blogs, you also can easily manage them and switch to them from within the app.