It can only be changed by editing the mediawiki page of the message box. Only admins can do this, but you can add a fake message to your user page, like the one i created in my user page. If you want, i'll give you the code for making one of those...
Penguin-Pal (talk) 18:32, January 4, 2012 (UTC)
Flex! I like ur new icon :) --CHOWDER2000The Puffle Dude! 20:25, January 9, 2012 (UTC)