Bah, no, don't put it into your /usr/share/...

Put your own icons into /home/user/.local/share/...

So instead of:
/usr/share/icons/hicolor/64x64/hildon/
use:
/home/user/.local/share/icons/hicolor/64x64/hildon/ (you'll probably have to create this directory the first time around).

The advantages of this are two-fold:
1. It won't delete the original icons, so if you ever want to revert, you don't have to reinstall your icons or whatever.
2. It'll override whatever your themes use as icons - so if your theme includes custom icons, there's no chance that categorize will use the theme's icons instead of your own. (The ones in /usr/share/icons... are the system default. Themes override system defaults, and icons in .local override those.)
3. It survives fiasco/firmware reflashes - so unless you're reflashing the ENTIRE device, including eMMC, you're not going to lose your custom icons.