From the GNU GPL FAQ:
Can I use the GPL for something other
than software?
You can apply the GPL
to any kind of work, as long as it is
clear what constitutes the “source
code” for the work. The GPL defines
this as the preferred form of the work
for making changes in it.
It is probably debateable, but I'd say whatever format you use to edit the image before saving would probably constitute "source code".
However, if you use pure GPL, that would render any program that links in your image in its executable GPL. If you don't want that, I'd suggest modifying this standard font exception to say "image" instead of "font":
As a special exception, if you create
a document which uses this font, and
embed this font or unaltered portions
of this font into the document, this
font does not by itself cause the
resulting document to be covered by
the GNU General Public License. This
exception does not however invalidate
any other reasons why the document
might be covered by the GNU General
Public License. If you modify this
font, you may extend this exception to
your version of the font, but you are
not obligated to do so. If you do not
wish to do so, delete this exception
statement from your version.