Change the System Messages with Icons
Fragment of a discussion from Project:Support desk
There's no universal answer: some messages accept the usual [[Image:Foo.jpg]], some don't. A few even accept raw HTML. If you can't achieve the desired result by modifying a message, you can try doing it with CSS: many parts of interface are wrapped in divs or spans with a unique ID, so that images could be added to them with site CSS. Look at page source or use something like Firebug to see the ID.
