These settings are related for the thumbnails, which appearance depends on the selected skin. Here you will able to set some basic style settings.
More Settings / Thumbnail navigation
The Thumbnail navigation parameter has 3 options:
Disabled will switch off this feature. Hover means that the thumbnails only become visible when you move your mouse over the popup, while always option show the thumbnails permanently.
It is also possible to set the Thumbnail container width. You can use px values and percent as well, referring for the popup's width.
In this section you can set the width and height of the thumbnails.
Here you can set the amount of the opacity of the active and inactive thumbnails.