Specify Image Dimensions

If you are looking for config you can buy our Default+ skin here http://www.nimble3.com/shop/premium-defaultplus-skin-for-nimble-portfolio/

If you can modify the php file, then copy plugins/nimble-portfolio/skins/default/items.php to [your theme folder]/nimble-portfolio/skins/default/items.php and modify the thumbnail size 480x480 to your liking, in the items.php you copied to your theme folder.