Icons are cached in one very big file:
GTK+ can use the cache files created by gtk-update-icon-cache to avoid
a lot of system call and disk seek overhead when the application
starts. Since the format of the cache files allows them to be mmap()ed
shared between multiple applications, the overall memory consumption
is reduced as well.
Every theme should create a cache file on installation.
So for example - if you remove/rename the gnome icon-cache file:
sudo mv /usr/share/icons/gnome/icon-theme.cache /usr/share/icons/gnome/icon-theme.cache.backup
You can then regenerate the cache:
sudo gtk-update-icon-cache --force /usr/share/icons/gnome
In your case, you have added your application icons to hicolor - therefore you should force the regeneration of this cache file to pick up the new icons:
sudo gtk-update-icon-cache --force /usr/share/icons/hicolor