Not much help but I had a quick look. All your missing thumbnails have links with images where “src” is empty. Eg.
<img alt="rrecent photo of the dam area at Ian and Helen's property" src="">
l also note (as is my wont to quote Billy Connolly) that the one that does work (Evans Tree Bee 2016) link to some hefty sized images - 2.5 mb (me with only a 1mbps connection - you would think living close to a major NSW regional centre I could get better!!!)