@laurengrace, I found the “Other Options” was one of the menu choices for the NGG Gallery. From there find the “Styles” menu. Bottom left in that menu window for styles is a hyperlink that says “Customization Options” click on that then take the code that tizz offered up in his post and copy/paste at the bottom of the code that is shown. hit save and you should now have center aligned thumbnails.
Hope this helps. Worked for me.