jamesperet-website-2020/themes/james-theme/layout/_partial/content-thumbnail.ejs

<% var thumbnail %>
<% if(thumbnail == undefined) thumbnail = post.thumbnail_image %>
<% if(size == 6) thumbnail = post.cover_image %>
<% if(size == 12) thumbnail = post.hero_image %>

<div class="col-6 col-md-<%= size %>" style="margin-bottom: 30px;">
    <a href="<%= url_for(post.path) %>" class="thumbnail-link">
        <img class="img-fluid thumbnail rounded" src="<%= thumbnail %>">
        <% if(show_title == true || show_title == undefined) { %>
            <p><%= post.title %></p>
        <% } %>   
    </a>
</div>