Rather than disabling the thumbnail image clickthru, I had good results using Code Injection to set the clickthru URL of the thumbnail image to match the clickthru URL of the post title, which is the post Source URL. I used the following Javascript code.
<script type="text/javascript">
document.addEventListener('DOMContentLoaded', initBlogImageClickthrus);
window.addEventListener('mercury:load', initBlogImageClickthrus);
function initBlogImageClickthrus(){
var aPosts = document.querySelectorAll(".blog-masonry .image-wrapper");
var i;
for (i = 0; i < aPosts.length; i++) {
var sPassthroughURL = "#"
var oPassthroughLink = aPosts[i].parentNode.parentNode.querySelector(".passthrough-link");
if (oPassthroughLink) {
var sPassthroughURL = oPassthroughLink.href;
aPosts[i].href = sPassthroughURL;
aPosts[i].target = "_blank";
}
}
}
</script>