This is the mod I use to set Icons on Mobs. It's called Baud Mark, found at: http://wow.curse.com/downloads/wow-addons/details/baud-mark.aspx (http://wow.curse.com/downloads/wow-addons/details/baud-mark.aspx)
(https://forum.twguild.org/proxy.php?request=http%3A%2F%2Fmediabeta.curse.com%2FCurse.Projects.ProjectImages%2F5168%2F7032%2FBaudMark.png&hash=d0c8ad5b8c725157124ea4480ee03f968b96bec0)
It's fairly easy setup, it puts an option in your key binding's. Just set a key aside to bring Baud Mark up. For me, I use (~) to bring up Baud Mark. It pops up where ever your cursor is. It's not possible to change the icon's around, so Skull will always be in the middle.