I am not sure what are you looking for but if you want to put an image on a button, then here is the code. Basically you need to define an image and set that image to the given button.
Gtk::Button *btn=Gtk::manage(new Gtk::Button("DummyButton"));
Gtk::Image *ImageIcon=Gtk::manage(new Gtk::Image(Gtk::IconTheme::get_default()->load_icon("ButtonImage",128)));
btn->set_image(*ImageIcon);
btn->set_image_position(Gtk::POS_TOP);