The default icon names are:
up.gif - life is happy
down.gif - life is definitely down.
maybe.gif - life may be down
dep.gif - I couldn't reach it because of a path failure
The default icons are simply colored spheres, but I've provided some alternates which provide shape cues as well:
stop.gif (a diamond, actually, so it's easier to see) for "down.gif" bluesquare.gif (a square for "may be down") yield.gif (a triangle for "path failure")
Simply rename these, or any other images you prefer, to the appropriate name, and you'll be set.