Running the following trivial test case app

<snip>
#include <gtk/gtk.h>

int main (int argc, char **argv)
{
  gtk_init(&argc, &argv);
  gtk_icon_theme_load_icon(gtk_icon_theme_get_default(),
			   "weird_name", 24, 0, NULL);
  return 0;
}
</snip>

will look up the icon "weird_name.png" inter alia in the folders (checked with
strace)

$HOME/.icons/gnome/24x24/apps
$prefix/share/icons/gnome/24x24/apps

but also in

$HOME/.icons/hicolor/22x22/apps
$prefix/share/pixmaps/hicolor/22x22/apps
$prefix/share/icons/hicolor/22x22/apps

If new application icons are installed below $prefix/share/icons/hicolor (as the
 description for gtk_icon_theme_set_search_path() indicates), they will not be
found for size 24x24. Creating a symlink 22x22 -> 24x24 will "fix" the problem.
hicolor "stock" icons are taken from hicolor/24x24/stock. So obviously there is
a bug in Gtk when calculating the hicolor apps path.