Assuming the .swf is just a thin wrapper around the image file, it should be pretty easy to scan through the file looking for the PNG or JPEG magic numbers and extracting the files based on their own metadata. A competent programmer could write such a utility in an hour or so easy.
Things get a lot harder if the data is DRMed in there or compressed again (uselessly).
My guess is that someone has already done this simply because it is so easy.
