from your FAQ
As Coppermine is based on OpenSource software published under GNU/GPL (which allows the modification of the code, but disallows sales of the code, modified or not), you can not purchase a version of coppermine with or without the credit line removed. Read the file COPYING that comes with the distribution of Coppermine for details.
from the GNU GPL FAQ:
http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowMoney Does the GPL allow me to sell copies of the program for money?
Yes, the GPL allows everyone to do this. The right to sell copies is part of the definition of free software. Except in one special situation, there is no limit on what price you can charge. (The one exception is the required written offer to provide source code that must accompany binary-only release.)
Having searched the forum for this topic coming up before, I discovered:
http://forum.coppermine-gallery.net/index.php?topic=23720.0Which
somewhat explains this - as the original developer is out-of-reach, and you haven't assigned copyrights to a central foundation, you cannot dual license the product, with the commercially licensed version allowing removal of your tag line. (by the way, I believe this "you mustn't remove the ad line is a gpl violation in itself, but I'm not complaining about that really, it seems fair to me)
However, you could still technically sell the GPL version, so perhaps you might want to reword your FAQ, because the "disallows sales of the code" part is basically completely wrong