Since this thread has been resurrected of late; it is worth noting the variety of work-around solutions presented which are universal:
1) attach a non picture file with your picture (I use an rtf named 'ignoreme')
2) change the picutre somehow (zip it, etc)
3) Use a different mail client, or web-based system.
4) Pay for a plugin to prevent this behavior.
None of which fix the problem- which is that some email systems still see the attachment as encoded, rather than attached. The blame lies with the email providers and with Apple. I had not had this issue with OS X Mail until 10.6; but since 10.6- some email providers actually updated their system and started reading attachments correctly.
Also, Outlook 2010 can 'save as' even an inline picture. Macs can easily grab any image and save it.
So there is hope.