Which looks to me like what i meant - but maybe i misunderstand that.
No, you are right. My bad.
You may have to specify both the small size (nnnxnnn) and the lightbox size (nnnXnnn)
to center, add space at both side of imagename
{{gallery> image.jpg?300x250&lightbox&1000x600 }}
works on my installation. centered, 300x250 in page and 1000x600 (or just big I don't really know) in lightbox popup.