Using proprietary graphics in a free program

[Tord Romstad]

Hi all,

I am Tord Romstad, author of Glaurung, one of the strongest GPLed
chess programs. Recently, I ported my program to the Apple iPhone.  A
couple of days ago, I was contacted by someone who wants to make a
commercial chess program based on my program.  He understands that his
derived program must also be available under the GPL, and intends to
publish all changes he makes in my code, so everything is OK so far.

The problem is that he plans to hire a professional graphics designer
to draw the piece images and some other graphics, and to keep these
graphical image files proprietary.  He asks me whether the GPL allows
this, and I honestly don't know myself.  Does anyone here know more
about this?

Tord