First go to this web site on a computer (it may work on a DSi i haven tested it yet). Go to any chat room (preferably an empty one) and type /avatarsuperlist in the chat bar. You should come up with a big list of avatars. Pick one you like. Drag the picture into the chat bar and it will automatically put something in the chat bar that looks something like this "http://dsipaint.com/chatrooms/avatars_chat/milkshake.png" (the example is the milkshake avatar). Whatever is between the "chat/" and the ".png" near the end will be what you type in after "/avatar " For example for this one the avatar would be /avatar milkshake.

Thank You and good night!