Bei
gnome-look.org wird's so erklärt:
installation
1. add a "Main Menu" to your panel
2. run gconf-editor
3. navigate to /app/panel/objects
find a object_x with the "object_type=menu-object".
(my main menu is "object_8")
4. tick up "use_custom_icon"
5. change "custom_icon" value to the image path.
Hier gibt es passende Buttons. 😉