Constrain images to 16:9

static/src/scss/_blocks.scss

@@ -1,17 +1,30 @@
+@mixin center-block {
+  width: $embed-width;
+  margin: 0 auto;
+
+  max-width: $embed-width;
+  min-width: $embed-width;
+}
+
 div.block-image {
   figcaption {
     font-size: 85%;
+    margin-top: 5px;
+  }
+
+  figure {
+    @include center-block;
   }
 }
 
-.content > div:not(:last-child) {
+.content > div[class^="block-"] {
+  &:not(:last-child) {
     margin-bottom: $content-block-margin-bottom;
   }
+}
 
 div.block-embed {
   lite-youtube {
-    max-width: $embed_width;
-    min-width: $embed_width;
-    margin: 0 auto;
+    @include center-block;
   }
 }

static/src/scss/_variables.scss

@@ -1 +1 @@
-$embed_width: 85%;
+$embed-width: 85%;

website/common/templates/common/blocks/image-caption.html

@@ -1,7 +1,9 @@
 {% load wagtailimages_tags wagtailcore_tags %}
 
 <figure>
+  <div class="image is-16by9">
     <img src="{% image_url value.image 'width-1500' %}" alt="" />
+  </div>
   <figcaption>
     {{ value.caption|richtext }}
   </figcaption>