I’ve been reading this discussion with some interest: I have a similar problem using latest WP & latest NG and have not been able to make NG work as desired. I hope folks won’t mind too much if I attempt to tag on to the existing discussion since the issue is almost identical.
In my case I have a responsive theme layout which uses full page width with no sidebars; NG is set to use 3 columns.
Since there’s a mix of portrait, landscape and square thumbnails what I am seeking to achieve is to center the display of the images within each column and also center the captions [description field] below each thumbnail.
I’ve tried many different settings and combinations in the main gallery settings and other NG options and regardless of whether I use ATP on the page or use a shortcode [with or without the template=caption option] and also regardless of whether I use custom CSS, I simply can not get images to center but can get the captions to center, and even to wrap correctly when they exceed the column width. Images always align left.
In order to test that the issue isn’t WP theme related I have also tried resetting WP to one of the default themes [2013], with no other plugins enabled apart from NG. None of these changes make a difference to the layout.
In the custom css I have at different times tried float and other alignment settings for .ngg-galleryoverview, .ngg-gallery-thumbnail and .ngg-gallery-thumbnail-box.
I’d welcome suggestions for settings I should examine or change, or custom CSS I should enter. I’m afraid that right now I can’t offer a link to the site since it is behind a firewall and will not be published until these and various content issues are resolved.