App Store for free.
It's very simple to add an animated picture to your MMS. You can browse the collection of user-submitted animated pictures by date and popularity. Or you can search using keywords. Once you found the perfect picture, just tap a button to start a new message. Goodbye plain boring SMS text. Hello fun animated MMS!!!
In addition, anyone can create and upload his own animated pictures to share with the world at http://www.txtagif.com/.