Guys,
I just refactored the image_gallery into a new module called PDF_gallery but then I came unstuck because the real issue is the awful GD library class. I haven't give up on this though and plan to extend the GD library to a GDPlus (or something), the plus version using Imagemagik
With Imagemagik I can convert my PDFs and other file types to thumbnails.
In the meanwhile though I ran out of time and wrote a Coppermine page type to bridge to that gallery program (which can be extended to PDF and Video files) This works beautifully.