This setting is found under VPS: Image Field Settings. This setting must be specified if Thumbs view is selected in either Saved Pages View or Page Selector View. Here you specify valid image fields that will be used to create thumbnails representing pages in either Thumbs view.
Naturally, the image field must be linked to the selectable pages (via their templates). The image fields are stipulated by selecting image fields from an Asm Select dropdown.
Please note that these image fields must each accept a maximum of one image or be of type 'Single item'. Only one thumbnail per page will be displayed. This will be the first image found. The image fields are iterated in the order specified here.