Forums / Cotonti / Skins / Group icon ?

<img src="skins/{PHP.skin}/img/group-{USERS_DETAILS_MAINGRPID}.png" alt="" />

Just make sure you have corresponding images (group-5.png for administrators, in instance). Thats for user details.

{FORUMS_POSTS_ROW_MAINGRPID}

should work for forum posts.