I experienced the same issue - the problem appears to lie with the "content-disposition" field in the message headers. Apple Mail always seems to set this to "inline", regardless of what settings you change ("View as Icon", "Windows-friendly", "Attachments at end" etc.) and, for some reason, Outlook users are unable to save these attached images files.
Finally, I discovered http://lokiware.info/Attachment-Tamer which is an add-on for Apple Mail and which can force the content-disposition field to "attachment" so the receiving Outlook client is able to save the attachments as normal. I have literally just been testing this myself and it seems to have done the trick.
I am in no way affiliated with Lokiware and do not gain in any way from promotion of their product - I'm just a very happy user!
C.