So you'd like to replace the standard Gnome foot or Ubuntu icon on your main panel menu? It seems easy at first, but you may have trouble getting it to actually change... Then follow these steps, and you should be a happy camper with a shiny new menu icon :)
Foo/scalable/placesdirectory where your icon theme is. Note this could be under either the system location (e.g., /usr/share/icons) or your own ${HOME}/.icons/Foo.
gtk-update-icon-cache /usr/share/icons/Foowhere Foo is the name of your icon theme (as above).
killall -s HUP gnome-panelAnd don't forget to select your custom theme (or add your icons to an existing theme) in the Gnome Appearance config GUI.
If that doesn't make your custom icon appear, then something went wrong; either you don't have the right theme selected, or perhaps your icon isn't a valid SVG file. If you used a PNG icon at a specific size, then your panel size could be different than your icon size, or perhaps an existing SVG icon is taking precedence over yours. Remember, Google is your friend, along with your distribution's support forums, etc. You can also find many lovely desktop and icon themes, backgrounds, etc, on sites such as Gnome-Look.org, as well as other places...