actually i didn't bother .. but today i had some time to kill so i searched about that issue and found that it is a known bug since karmic and the solution is not that hard
here are the steps :
- Press Alt+F2 ... type gconf-editor and press run
- navigate to : /apps/desktop/gnome/interface
- and make sure that the menus_have_icons is marked
here is the gconf-editor screen
and you will have your icons back :)
i also noticed that this option brings icon to many menus in Ubuntu not just the system menu
here is the bug page: https://bugs.launchpad.net/ubuntu/+source/gnome-panel/+bug/442973
hope that was useful