Eh? Just turn the option on and remove the code from showthread.php that puts in the default icon. You will then get the icon on all the posts on the forumdisplay listing but not when viewing threads. In other words remove this from showthread.php
Code:
if ($showdeficon) {
$post[icon]='<img src="{imagesfolder}/icons/icon1.gif" border="0" alt="">';
}
and turn the option MasMind mentioned on.