[Plugin: NextGEN Gallery] Centre thumbnail vertically and horizontally

When I specify to have both portrait and landscape images included in the thumbnail list is there any way to center those within the containing “ngg-gallery-thumbnail-box” div?

Otherwise if I have images of varying sizes they end up looking a bit of a mess, whereas what I’d like is for the containing div to have a background colour and then the image centered vertically or horizontally i.e. have borders on the top or bottom if they are not the same as the div proportions.