Add more comments to explain thumbnail sizing

res/css/views/messages/_MImageBody.scss

@@ -28,6 +28,10 @@ limitations under the License.
 }
 
 .mx_MImageBody_thumbnail_container {
+    // Prevent the padding-bottom (added inline in MImageBody.js) from
+    // effecting elements below the container.
     overflow: hidden;
+
+    // Make sure the _thumbnail is positioned relative to the _container
     position: relative;
 }