[nivo thumbs=n nav=n] – means that no navigation controls will be displayed

[nivo nav=n thumbs=y] – still means that no navigation controls will be displayed since nav=n overrides thumbs=y

[nivo nav=y thumbs=n] With nav=y and thumbs=n you just get the navigation buttons

[nivo nav=y thumbs=y] With nav=y and thumbs=y you get thumbnail image navigation