how to do that??

[Admin]

In forumdisplay.php replace:

if ($foruminfo['allowposting']==1) {
  eval("\$newthreadlink = \"".gettemplate('forumdisplay_newthreadlink')."\";");
}

with:

if ($foruminfo['allowposting']==1 and ($foruminfo['forumid']==4)) {
  eval("\$newthreadlink = \"".gettemplate('forumdisplay_newhacklink')."\";");
} elseif ($foruminfo['allowposting']==1) {
  eval("\$newthreadlink = \"".gettemplate('forumdisplay_newthreadlink')."\";");
}

where 4 is the ID of the forum you want this for, and 'forumdisplay_newhacklink' is the template that has the new icon.