One thing that bothers me with NextCellent Gallery (NCG) is the missing option to fit the images into the screen (using large images). In order to enable this you have to use wp-jquery-lightbox plugin and disable the effects in NCG. In jquery settings you can check the option "Big images fit into screen" or so (I have the german version: "Große Bilder in Bildschirm einpassen").
Then the images will be fitted into the screen. This is a workaround as long there is no option in NCG available.