14.7. Modifying the PHP-Nuke theme icons

You are not allowed to use PHP-Nuke's own icons (found in phpnuke.org: Topics). From the PHP-Nuke FAQ:

Can I copy and use your topics graphics?

Absolutely No. All the topics graphics has been made by me and have copyright. You can't copy, modify and/or use those graphics. You can only use "phpnuke.gif" topic image to setup a topic in your web site that refers "only" to PHP-Nuke. Will be exceptions to this rule for PHP-Nuke community's sites. If you run a community support site (for PHP-Nuke ONLY), drop me line. You'll need my written permission to use them. Be fair and don't play with my hard work.

Thus, you must use your own icons. You will have to either use icons that are royalty free, or licence some for use on your site, or create your own ones.

To create your own PHP-Nuke icons, you can use some of the tools available specifically for this purpose, e.g. The Topic Dude.