When I'm linking a url to showgallery pic I don't include - &cat=1215 www.domain/gallery/showphoto.php?photo=1215
This is something I've been doing all along, hoping that I'm not breaking anything, but if a pic is moved to a different cat the link still works.
(IMG tagged links need cat# correction if moved)
If members enter a link in a post at the forums I usually check it out and change/remove any cat details.
If I'm directing a link to the Members Gallery I'll add &cat=500 http://www.domain/gallery/showphoto....o=1215&cat=500
but not include - &page=4 - because this might change.