Thanks, both of you!
I entered an image in the auto-correction and linked it to a shortcut: I type (e.g.) tt and press enter, which triggers my signature (Tim Tayler). So first of all it's not an image, therefore I can't use what you suggest, macropod.
I was hoping to find either something (e.g. a macro) copy protecting all images inserted in the document or a copy protection that still allows users to edit the file...