I have an application that I am considering making Open Source with the GPL. All code was written by me, but I am using proprietary images and sounds. Do I have to make these images and sound files freely available to be covered by the GPL? I'm sure this question is addressed somewhere, but I couldn't even find the word "image" in the entire GPL faq... http://www.gnu.org/licenses/gpl-faq.html

I'm not sure if this helps, but when ID made Quake 1 GPL they only released the source code under the GPL. The levels/art/sound were all still proprietary, so you could download the source for free (and compile the binaries) but you still couldn't play the game without buying the CD containing all the art assets.

The GPL is written and designed to be applied to source code, not to images or other data. There is debate about if it can actually apply to non-source content.

You could investigate the GNU Free Documentation License since it is designed more toward the data side of things.

One of the requirements of both licenses is that the file contain certain copyright statements. Usually those statements aren't incorporated into images and audio files. I'm not sure how that would be handled if challenged in court, but it is an important detail that is often overlooked.



As far as distributing the code for free but the content at a cost, there is no problem with that, provided that the data is not strictly necessary for the system to run. The easiest way to ensure that it isn't strictly necessary is to have both a free and a non-free data set.