Are you looking to do something like this:
That's my standard for emailing or posting on forums. It's done with the excellent plug-in BorderFX.
There is no better way to add borders (or text) with Aperture's built-in features.
With apologies for _seeming_ to beat-up on you -- not my intent -- that's not really "something that works differently than you expected". As such, it is an excellent candidate for a separate thread. .
Message was edited by: Kirby Krieger