By default the image displayed for a page in a link to that page is the first image that can be found on that page, or on a child page.

The @icon directive can be used to override the link image used, or have a link image where there is no image on the page itself.

So for example the Computing page has:

  @icon		imac.jpg

No image is displayed on that page, but an image is displayed for the page on News. Unless there are images to display for every page in an index a text index will be displayed instead of an image index.