Possible to do but my main issue is it is going to create a lot of issues for other users.
I can use the single image and have it multiple galleries but have to think about performance, if you delete the image in one gallery it will delete it completely.
Handling of previous/next images.
Ah, okay I understand. And I understand also why you copy it really to an other cat. So I think we keep it the way it is now. No problem. We'll edit the SSI to display only the 'default category' so the image is displayed only once..
Thanks for your answer!! And have a nice weekend!