[Plugin: NextGEN Gallery] center image into box

i have a problem with the gallery layout…

i set Thumbnails options:
Width x height (in pixel) = 245×245
Set fix dimension = not selected

and the css:
.ngg-gallery-thumbnail-box {
background-color:#121212;
float:left;
height:246px;
margin:1px;
width:246px;
}

how can i center the image into the thumbnail box?
(orizontal and vertical)

you can see my page here:
http://www.ilmobileclassico.it/prova/