### INSTALL ###
# If you haven't done so already, add a "Main Menu" to your panel by right-clicking on an empty area on the panel and selecting "Add to Panel". There are two types of menus there - you want to pick the one represented by a single icon of the Ubuntu logo (or whatever your Linux distro uses as a default image). Also remove the Applications/Places/System menu from your panel if you have one of those set.
# 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 you'll 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/.images/image.png". The graphic should update immediately.