>I think such "sections" belong in different files and the document should
>essentially be a directory. this can be put into a tar-file for transport.

I have to say that I really agree with this approach.  Then the documents
can still be valid XML, just containing an XML tag that points to the
content to be included.  This should confuse parsers a LOT less than a
series of XML sections separated by binary chunks.  It will also make it
much easier and more efficient if the user wants to, say, look at or tranfer
a preview file that doesn't include all of the final graphics.  The program
could just display an empty, correctly sized box where the binary section
would go.
--JZ