Place in usr/share/icons/elementary/ and update your icon cache to apply.
To update your icon cache, run the following in terminal:
sudo gtk-update-icon-cache --ignore-theme-index -f /usr/share/icons/elementary
Included 16x16, 22x22, and 24x24 sizes (22x22 shown in picture).
It's been driving me nuts that my custom panel icons were just a touch lighter in colour than those bundled with the OS, so I finally got around to correcting it.