To set the custom menu graphic, you need to open the [Configuration Editor] under the [System Tools] menu - or for you hardcore types, [gconf-editor] at the command line. Go to [apps->panel->objects] and youll see a series of [object_x] subitems. Click through them until you find the one where the value for [object_type] is [menu-object]. Check the [use_custom_icon] option and then set the [custom_icon] value to the path to your custom button graphic, i.e. [/home/username/Pictures/ubuntumenu.png]. The graphic should update immediately.