I don't like Gnome's default logo for the main menu because is too small and hard to find among the icons on the panel.
So I did some alternatives that  you can easly replace with:

sudo mv /usr/share/icons/hicolor/48x48/apps/distributor-logo.png
/usr/share/icons/hicolor/48x48/apps/distributor-logo.png.bak
sudo cp  gnome-logo-xxx.png /usr/share/icons/hicolor/48x48/apps/distributor-logo.png
killall gnome-panel