GPL has got me confused and I'm hoping that someone(s) can shed light on this.

For example, if I create a commercial theme for WordPress, I understand that the PHP code is GPL'd by default -- at least this is my understanding.

But what about the stylesheets, sample images, gradient images, icons, and layout? Are those also put under GPL by default, or can I specify a different license for those elements?

The theme won't be distributed *with* WordPress, but it really doesn't do anything *without* WordPress.

Thanks for any insight!