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:
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.