Thanks for the reply.
I can already create audio or video files no trouble, the m4a format I'm struggling with allows for a series of still images to be displayed at selected points within an audio file. It's only really used by itunes and ios devices. I'm using it to give to kids as an audio of the theory notes, with diagrams etc shown at relevant points. Essentially the same as a video file, but a far smaller file size for downloading and storing on their device.
As I said, I can now create the enhanced podcast file no problem, and it plays in itunes and on ios devices perfectly - I just want to allow them to see the same file on a web site without having to re-encode it as a video file first. Not an insurmountable problem, just an irritation.