I use a standard mainmenu that is connected to a TImageList with 22x22px-icons. Icons are well shown with Gtk2 (Ubuntu 10.4), and correct on KDE 4 with Gtk2-libs (openSuSE with latest updates) but are replaced by simple squares on Ubuntu 11.4 with Unity or Gnome3. Maybe 16x16 works, I didn't try (Gtk3 runs only on my office PC as backup system).
Scrollbars are fine on Gtk2, on Gtk3 and KDE they do not have smooth scrolling by thumb, page up/down is okay.