Comment 5 for bug 488242

I don't know very well how it works, but I know that the standard place to put specific icons is /usr/share/appname/icons/hicolor/YYxYY/ , where YYxYY is the size of the icons (16x16, 22x22, 48x48, …).
I guess that then, you need to append this path to a kind of GTK icon path, I think you should be using gtk_icon_theme_set_search_path (see http://library.gnome.org/devel/gtk/2.18/GtkIconTheme.html#gtk-icon-theme-set-search-path ) , though I'm not completely sure, I never wrote a GTK application ;⋅).

And of course, to get the right icon, you'll need to get it via gtk_icon_theme.load_icon (see http://library.gnome.org/devel/gtk/2.18/GtkIconTheme.html#gtk-icon-theme-load-icon ).