You can change the icons without having to recompile. The lines below can be added wherever you are implementing the roster class. Here I have changed the icon that is displayed for switching a users role to audiance in the sub menu when you role over a user in the roster list. Just make sure the file is in your assets dir
[Embed(source="/assets/hostRosterIcon.png")]
var icon:Class;
Roster.MENU_AUDIENCE_USER_ICON = icon;
If you want to change the menu icons that are displayed next to the users name you will want to look into creating your own itemRenderer for the roster class.
-chris