FlashChat allows you to send pictures and messages quickly through an open chat channel on your iPhone, using the contacts list you already have and a public directory of users to connect you. The result is a proprietary replacement for your SMS app, but one that requires people to have the same app installed. While there are some benefits of using this instead of SMS, including the cost (FlashChat is free!), the app doesn't offer quite enough new features to warrant complete replacement of your other messaging apps.
Installation and setup for FlashChat is fairly straightforward. Create an account and, when prompted, send a text message to the developer's support team to verify your phone number. You can also import your contacts at any time if they are on FlashChat or invite everyone on your contact list individually to sign up and start chatting with you. Where FlashChat stands out from other free messaging apps is that it integrates both private and public messaging. You can add friends but you can also "Discover" people in the directory and chat with them, directly. You can send a message to anyone publicly or you can add them as a contact and chat privately. The interface is a bit basic -- you're browsing through long lists a lot of the time -- but it works well for what you're doing.
If you're interested in a free alternative to your existing messaging app(s), consider FlashChat. It's not a perfect messaging tool, but it's free to use, has a decent user base, and the messages move surprisingly quickly between users.