Update image file type

_includes/thumbnail_podcast.html

@@ -7,7 +7,7 @@
         >
       {% else %}
         <img
-          src="{{ site.img_url }}thumbnails/podcast/{{ episode.custom_thumbnail_name }}.jpg"
+          src="{{ site.img_url }}thumbnails/podcast/{{ episode.custom_thumbnail_name }}.png"
           style="max-width: 365px; max-height: 223px;"
         >
       {% endif %}