Improve accessibility

_mixins.pug

@@ -3,7 +3,7 @@ mixin responsive_image(alt, thumb, fallback, urls, sizes, aspect_ratio)
     .responsive.placeholder(style=`padding-bottom: ${aspect_ratio};background-image: url(${thumb})`)
       noscript
         img(src=fallback, sizes=sizes, srcset=urls, alt=alt)
-      img(data-src=fallback, sizes=sizes, data-srcset=urls)
+      img(data-src=fallback, sizes=sizes, data-srcset=urls, alt=alt)
 
 mixin theme_card(id, name)
   a.card(href=`/assets/images/${id}.png`)