[Logician]

Not exactly what you want but similiar:

edit newthread.php, find:

PHP Code:

 $iconid = intval($iconid); 


After that add:

PHP Code:

 if ($iconid ==0 AND $bbuserinfo['usergroupid']!=6) 
{
eval("standarderror(\"".gettemplate("error_nomessageicon")."\");");
exit;
} 


Create a template named "error_nomessageicon" and insert your error message there. If you have more than 1 template sets create it in all sets.

Basically it will give the error message if new thread poster did not choose a message icon and he is not an admin. The difference from your request is: This is not popup.