hexo-theme-icarus/layout/_partial/post/thumbnail.ejs

<a href="<%- url_for((post.link?post.link:post.path)) %>" class="thumbnail">
  <%
    var s = "";

    if (post.thumbnail){
      s = post.thumbnail;
    }else if (post.banner){
      s = post.banner;
    }else if (post.photos && post.photos.length){
      s = post.photos[0];
    }else{
      var img=/\<img\s.*?\s?src\s*=\s*['|"]?([^\s'"]+).*?\>/ig;
      var result = post.content.match(img);
      result = img.exec(post.content);
      if(result != null) {
        for(var i = result.length -1; i; --i){
            s += result[i]+"\n";
        }
      }
    }
    if(s.length > 0){
      if(config.post_asset_folder){
        var pattern = /^[\\{0,1}\/{0,1}]([^\/^\\]+)/,
            pattern_ = /([^\/^\\]+)/;
        if((ret = pattern.exec(s)) != null) {
          if(ret[0].length == s.length)
            s = config.root + post.path + ret[1];
        } else if ((ret = pattern_.exec(s)) != null) {
          if(ret[0].length == s.length)
            s = config.root + post.path + ret[1];
        }
      }
     %>
    <span style="background-image:url(<%- url_for(s) %>)" alt="<%= post.title %>" class="thumbnail-image"></span>
  <% } else { %>
    <span class="thumbnail-image thumbnail-none"></span>
  <% } %>
</a>