Gallery Sort – Latest on Top?

What is the shorthand code to make the gallery show the latest additional pictures on top rather than on the bottom? On my site, I’m constantly adding new images to an existing page/post, and instead of having the new images show up at the bottom, I’d rather it show up on top first, and from there, go by latest addition first, oldest last.

I know you use [gallery] to show the gallery, but can anyone show me how to make it do what I want — newest addition to the gallery on top, oldest at the bottom?

Thanks.