I can see not deleting if linked but rename should be global if in the image is in fact linked to multiple trees, I think.
Unfortunately not possible. Yes, we could change the physical file name on the server, but we cannot change the GEDCOM data linking it to each (of multiple) GEDCOM files. The entire editing system is designed to prevent any (accidental or otherwise) modifications to one family tree when working on another.
The code must cater, for example, for sites that have multiple admins, each responsible (only) for their own tree, but possibly sharing media objects. They might not even know other trees exist.