Found solution.
http://wpbugs.com/2013/05/nextgen-gallery-thumbnail-centering/
Go into the nextgen “gallery” menu in wordpress, click on “other options”, then “styles”, then “show customization options” and paste the code underneath the text that’s already there.
I just copied/pasted the bottom three parts, since those are the ones that actually cause the thumbnails to center.
I had to adjust the width and height values in the code since I had already customized my thumbnail sizes to 300×300 in the Gallery Settings/NextGEN Basic Thumbnails menu.
It worked for me: http://www.highdesertmultimedia.com/sample-work-posters/
