When checked the index (TOC) page will include thumbnails. Each folder must have a thumbnail image selected. Thumbnails by default are used with the thumbnail bounds setting and with 'HiDPI' (2 * size) forced. Thumbnail variants are not supported.
Thumbnail shape and position
There is an option to have thumbnails float to the left, if unchecked the thumbnails will be arranged vertically.
Thumbnail images can be shaped differently to how the thumbnail bounds are set - the default choice: constrain - the aspect ratio limited to 0.5 to 2 to limit the width of panoramic images;
Fixed height - The aspect ratio is kept as much as possible but each thumbnail has the same height;
Square - each thumbnail will have the width and height set to the maximum thumbnail height bounds;
Thumbnail titles can be positioned above, inside or below the image. Titles will be constrained to fit within the thumbnail 'box'.
If the titles are inside, the title can be as long as will fit within the image bounds before being clipped. The title will only appear when the mouse is over the thumbnail.
If titles are above or below the image they will be on one line unless the checkbox 'Constrain to one line is unchecked'. When unchecked the title space expands to allow for two lines of text.
When checked thumbnails will appear below the 'previous' and 'next' item labels, in the top and bottom navigation areas. The thumbnail height can be adjusted in the range of 10 to 100 percent of the thumbnail height. On screens with a width below 768 pixels the thumbnails will be vertically stacked.