Hey,
you didn't say which environment this struggle is happening in, so I'm just going to assume it is GNOME, and gnome-settings-daemon is running. In that case,
gsettings set org.gnome.settings-daemon.plugins.xsettings overrides "{'Gtk/MenuImages':<1>}"
will do what you are asking for. However, you might find that many menuitems just don't contain icons anymore - in that case, no setting will bring it back.
