Forums / Cotonti / Core Labs / Archive / Subforum icons

try this:
yoursite/admin.php?m=forums&n=edit&id=1(click on some subforum name to enter its properties)
there you have an Icon field in which you can insert the link to the picture
and in the forum.topics.tpl insert

{FORUMS_SECTIONS_ROW_ICON}

in the :
<!-- BEGIN: FORUMS_SECTIONS_ROW_SECTION -->
....
<!-- END: FORUMS_SECTIONS_ROW_SECTION -->