You can't do this in GarageBand. You ought to be able to do it in Posterous, and it's possible you could do it in Feedburner (though it would be better to do it in the original feed) but I can't advise on either. Posterous is producing a very messy feed, even though it does work up to a point, and I have a gut feeling that they really don't know what they are doing, so whether this facility is available I don't know.
As you have now got the Store reading the Feedburner feed it's actually not that difficult to move to another podcasat creation service as all you have to do is tell Feedburner. However, the place to start is Posterous's episode creation pages, and their Help and/or Support (if any).