It isn't something I'm really interested in (I don't remember to use them) but I've info on LJ about them.

Have a look at the second half of this page: http://www.livejournal.com/doc/guide/moodicons.bml

Looks a bit complicated but if you have two windows open, one with those instructions and the other you actually try the stuff out in, at least you can refer back to the instructions. That's how I learned how to do custom friends groups and some of the stuff I've done with my LJ.