Thank you for your post, and I apologize for the late reply.
This should not be a problem, but then again, I don't know what the GD graphics library interfaces with PHP. You will have to try it.
If you have tried this and it works, please post here so others can benefit.
GD is not in either the FileMaker PHP or Apple's PHP installation in 10.5. It is included in Apple's PHP in 10.6.
Our web site integrates FMSA data via XSLT stylesheets which we have had in use since before FM supported PHP. I have since been gradually working up to using PHP since it appears to be easier and more commonly used. My experience is this:
We have a 10.5.4 server which has the Apple default PHP updated with GD using procedures found at http://osx.topicdesk.com/. We added the GD at the time 10.5.4 was newish and what I did is long out of my head. I could not even begin to help with that integration in and of itself.
The test pages in the Server Admin Console from the sample database DO show and interact as expected. We use GD to run a PHP slideshow of photos on the same page as our FMSA data but do not use GD directly on the FMSA data. It seems to me that you should have no problem based on the test pages I see, but, I cannot be sure since I have not actually used PHP with FMSA yet. At a minimum try to update the Apple PHP see if you can get that to work with your site. If not, you can always revert to the FileMaker PHP.
Hope this helps.