iBook covers in non standard size

[splinters]

I have quite a few picture books on the iBookstore and have had some trouble with lulu saying my covers are not the right size. Most were fixed by either stretching the image or adding white bands to the covers where they are not ebook size (some are from landscape or square style books).
Is there some way of ensuring you cover looks correct in your chosen format?
I though of using a PNG-24 format with transparency but not sure it will work.

Any advice on how I can get, say a square cover, to look correct on the bookshelf and pass the publishers standards. They don't look too good at the moment stretched or with the white bands.