To have a GIF as avatar you need to buy VIP, once you've got VIP you can just upload it in your user cp. You can find all about VIP in this FAQ thread: http://forum.toribash.com/showthread.php?t=465052

Otherwise you can buy a gif as avatar in the nabi profile customization store here: http://forum.toribash.com/showthread.php?t=446814
However this is a 1 time payment so when you want a different gif as avatar, you'll need to buy it again.