Fix image gallery

Adjust the aspect-ration of gallery images to actually be 4:3.

Render image credits by using the picture.credits-variable instead of picture.image.credits.

sass/_utils.scss

@@ -9,3 +9,7 @@ figure.image > figcaption {
   width: 100%;
   opacity: 75%;
 }
+
+.content .gallery figure {
+  margin: 0;
+}