I did what you suggested, here goes in more detail:
1. I use processFormSubmission for getting the attacment out of the document (might be possible in another way though).
2. I loop through the list of attachments to get to one I need
3. When I find it I use "getDocContentBase64(doc)" method and saves to string
4. I store the string in my xfaform (in a normal textfield)
5. On the form I have some simple code on initialize that sets the data from the textfield to be assigned to the image field for the image.
Thats it...simple eh :)
PS: There is a glitch in the Help for LC 8.2, it says that there is a " getDocFromBase64(aString)" method available, but it is not there...