said by DontTrustFam :
Is there any way to disallow destructive editing but allow view and save as access? Setting user permissions wouldn't help as they'll be accessing in through a NAS that doesn't have security controls for read-only.
Maintain a master copy offline (as far as the NAS is concerned) and allow full write to the copies. If you are happy with the modifications, overwrite the master copy or maintain the edited content as a revision / archive file.
Pictures are small, respectively. There is absolutely no reason why everyone should not have at least two copies of every image that is worthwhile saving.