swfmill

The open source tools around flash are growing:
swfmill can convert swf to xml and backward. I just tested it with a simple jpeg in a swf and it worked as expected.

From the README:

swfmill is a tool to process Shockwave Flash(TM) (SWF) files.
It can convert SWF from and to an XML-dialect called “swfml”,
which is closely modeled after the SWF file format.

It also provides a libxslt-based XSL transformator that
supports an extension (“swft”) which helps with generating
IDs for SWF objects and can import an SWF as XML using an
XPath command (swft:document()).

As a simple application of such functionality, swfmill can
pack together a bunch of media files (currently only jpeg
and other SWFs) into an SWF as “library objects” for your
attachMovie() pleasure.

swfmill is open source (C++) and published under GPL license.
The author has focus on building container movies for mtasc.