How to add icons to gnome menu items?

[Andrew Robinson <awrobinson cox net>]

I have some items in my gnome menu that do not have icons associated 
with them. To associate an icon, I tried right-clicking on the item, 
selecting "Properties", clicking on the icon button, then selecting the 
icon from the list. When I close the "Properties" dialog, I get this 
error message:

  Cannot save changes to launcher

  Details: Error writing file
  'applications:/Office/MoreOffice/jpilot.desktop':
  Read-only file system

I get this even if I am logged in as root. How do I make this change? Is 
there a file on the system that corresponds to 
"applications:/Office/MoreOffice/jpilot.desktop"? More generally, how 
does the "application:/..." map into the file system? Is it something I 
can edit? Even more generally, is there a way to add applications to the 
gnome menu by editting files?

Thanks!

Andrew Robinson