I have created custom latest blog template. But I can't show cover images in thumbnails.

Cover image should be here:

I have written following code to show the cover image:

<t t-set="properties" t-value="json.loads(post.cover_properties)">
  <a class="o_panel_cover" t-attf-href="#{blog_url('', ['blog', 'post'], blog=post.blog_id, post=post)}" t-att-style="background-image: #{cover_properties.get('background-image')};"> </a>
</t>
<div class="panel-heading mt0 mb0">
  <h4 class="mt0 mb0">
  <a t-attf-href="#{blog_url('', ['blog', 'post'], blog=post.blog_id, post=post)}" t-field="post.name"></a>
  <span t-if="not post.website_published" class="text-warning">
   <span class="fa fa-exclamation-triangle ml8" title="Unpublished"/>
  </span>
  </h4>
</div>

After writing the code image not loading and it shows like this:

How can I show the image?