Adobe Acrobat Ridiculous Document Protection – How to prevent an image from being copied from a PROTECTED document?
You shouldn't rely on that feature for serious protection of your document. It is simply not possible to restrict access to the image by third party software if you still want the user to be able to view it. I.e. it is not really a bug in Preview, it is a limitation in what's actually possible for Adobe to enforce.
You could take a screenshot of the final, composed product and use that in the PDF instead of your original material. That way the image will be blended together with the rest and cannot as easily be separated again.