Tumblr posts don't show images when viewed from homepage, only when I enter the post does the picture show?

Question

I have a problem with my tumblr photo posts showing correctly on my blog. When you are on homepage and scroll down to the "portfolio-post" section, even though the posts have images, they don't load on homepage (an example).

They DO LOAD once you enter the post (an example).

Can someone please review this code:

   <!-- start portfolio -->
    <section class="page-section portfolio-wrap">

      <div class="row">
        <div class="text-center">
          <div {block:SearchPage}style="display: none;"{/block:SearchPage}{block:TagPage}style="display: none;"{/block:TagPage}>{block:IfPostAreaTitle}<h3>{text:Post Area Title}</h3>{/block:IfPostAreaTitle}</div>

          {block:TagPage}<h3>{lang:Posts tagged Tag}</h3>{/block:TagPage}
          {block:SearchPage}<h3>{lang:Search results for SearchQuery}</h3><p>{lang:SearchResultCount results}</p>{block:NoSearchResults}<h3>{lang:No results found}</h3>{/block:NoSearchResults}{/block:SearchPage}
          <div class="divider"></div>
        </div>

          <div class="large-12 columns portfolio">

            {block:Posts}
            <article class="large-3 medium-6 columns">
              
               {block:Photo}
               <div id="portfolio-item" style="background-image: url('{PhotoURL-500}');">
            
                    <div class="post-info">
                        <a href="{Permalink}" title="{lang:Permalink for this post}"></a>
                        {block:Caption}{Caption}<br>{/block:Caption}
                        {block:IfShowPostDate}{block:Date}<small>{lang:Posted on DayOfMonth Month Year}</small>{/block:Date}{/block:IfShowPostDate}
                    </div>
                </div>
               {/block:Photo}

               {block:Photoset}
                <div id="portfolio-item" {block:Photos}style="background-image: url('{PhotoURL-500}');"{/block:Photos}> 

                    <div class="post-info">
                        <a href="{Permalink}" class="post-info" title="{lang:Permalink for this post}"></a>
                        {block:Caption}{Caption}<br>{/block:Caption}
                        {block:IfShowPostDate}{block:Date}<small>{lang:Posted on DayOfMonth Month Year}</small>{/block:Date}{/block:IfShowPostDate}<br>
                        <small>{PhotoCount} {lang:Photos}</small>
                    </div>
                </div>
               {/block:Photoset}