[Plugin: NextGEN Scroll Gallery] Centering thumbails

I’ve installed NextGEN and Scroll gallery, and I’m trying to work out how to center the thumbnails under the main image. I’ve only got 4 images in a gallery, so it looks a little untidy to have those 4 off to the left.

Been trying for a couple of hours, with no success. Any advice? Thanks.

http://ww.wp.xz.cn/extend/plugins/nextgen-scrollgallery/