Thumbnail photo problem

Hello. I just uploaded and published some thumbnails that enlarge when clicked on. But in editing the text on the page I unintentionally clicked on that chain/broken chain icon and made (somehow!) one of my thumbnails no longer enlarge when clicked on (when I hit save and tested it). In experimenting in trying to get it to enlarge, I (somehow!) made another of the thumbnails no longer enlarge! Now two, and I better stop. Could someone please give me a very basic step by step of how to get them to go back to where they’ll enlarge? I assume the first step is to click on edit….. Thanks!