Hmm shouldn't have posted before doing my homework:
There are at least three solutions to this problem:

* Embed the binary data directly in the XML document using the CDATA tag.
* Refer to the binary data using a URL.
* Encode the binary data into a text-based format that can be set as the contents of an XML element.
http://uk.builder.com/manage/work/0,...0268607,00.htm (and many more if you google around)