You could embed the image into a background image and then use Format |
Background | Fill Effects | Picture to insert the background, complete with
the "locked" image. This will make it at least difficult for users to
modify.
Or another trick is to make a copy of the slide, remove anything on the slide
that should remain editable later, export the result to an image file. Then
delete everything from the slide, insert the image onto the slide and finally
copy onto it anything that should be editable. Whew.