[Plugin: NextGEN Gallery] Centre images in imagebrowser

I very much like the Nextgen gallery and I have integrated it in my website. Because the vertical pictures were shown too big (even larger than I uploaded them) I removed the width: 100%; in the CSS file. I now have the problem that thepictures in the imagebrowser are outlined on the left on the screen where I want them centered. You can check
here what I mean: http://www.johnstuij.nl

Any help will be much appreciated!
Cheers,
John