Foren / Cotonti / General / Icons

I guess you need to use the tag {FORUMS_POSTS_ROW_MAINGRPICON} for it.
I am not sure.

search in forums.post.tpl for {FORUMS_POSTS_ROW_MAINGRPSTARS} and replace it by {FORUMS_POSTS_ROW_MAINGRPICON}

EDIT: I tested it, and it works.