Shoutbox

The defaults don't change for a very good reason: they're standard. When they are sent over the network, they are sent as the text shortcut. Then, at the other end, they're converted to the emoticon. Reduces bandwidth load.

Of course, with custom emoticons, the file has to be sent too, but it's cached after the first send, to avoid having to send it again all the time. Again, saves on bandwidth.