A more detailed explanation on chatroom avatars?

I'm kind of confused as to how to use a chatroom avatar. There isn't a lot of details given to help you use one either. Can someone tell me how to use one?

Roll with the punches... and the Katamari!

Although some avatars you have to have a code.

Falling like the rain , But Your the best drop EVER! 032611<3

Does this have anything to do with the Chat Command in My Settings that I totally don't understand?

Catch my drift.