Paste the following css to your style.css
.ngg-gallery-thumbnail-box { margin: 5px .5%!important; width: 32.3%!important; }
.ngg-gallery-thumbnail a { display: block; overflow: hidden; }
.ngg-gallery-thumbnail a img { border: 1px solid #8eb83b!important; width: 95%!important; padding: 2%!important; margin: 0 auto!important; }