Sparta try this

dialog control {
title "Control Panel (/cp)"
size -1 -1 215 155
option dbu
icon $mircdir\icons\ico.icl, $+ %icon, 0

Try placing the icon call after the Size and option. I dont have the ico.icl library For me it workes with my ICON library

Just a thaught I also gave it an ID of 0

Anyway good luck