The default installation of Wordpress now features thumbnails in the search and category listings. You just have to enable them. The first step is to select 'Gallery' in the new 'format' box. then select 'Set featured image' (all the way at the bottom right corner) to add the image that will be the 'thumbnail' (although it is a bit larger).
Perhaps tomorrow I will show you how to set up pull down menus also without any coding!