Well... this issue is a bit complicating. According to the GPL, if you make modifications to the original PunBB code, which always contains a copy of the GPL itself, then you will have to release the modified code under the GPL, unless of course you have done so for your site only and for your personal use and thus are not willing to release the modified software.
On the subject of styles or graphics now. If you release a completely new style, not based on an original GPL'd style in any way, then you may distribute it as you see fit. Again, as long as it has nothing to do with PunBB code which is licensed under the GPL! The same goes for graphics as CodeXP correctly mentioned. If you create your own graphics, and I mean your very own graphics (!), then you may also keep them for yourself and let people know that you are not willing to distribute these graphics under the terms of the GPL!
To cut it short. If you modify GPL'd content, whether that is PunBB's code, a GPL'd PunBB style etc, then you have to re-distribute it under the terms of the GPL, if of course you decide to do so!
You may read the whole text of the GNU GPL: