How to display page thumbnail on search results ?

Hello

Even when a featured image is set on a page, if this page appears on a search results page, no thumbnail is displayed (unlike posts).
See example here : http://reseaux-chaleur.cerema.fr/?s=am%C3%A9nagement

Any idea how to fix this ? (hopefully with custom CSS or something that doe not require modifying too heavily the theme files).

Thanks !