Add ability to have a fallback image if the primary no longer exists.

layouts/shortcodes/image-with-fallback.html

@@ -0,0 +1,17 @@
+{{   $url1 := .Get "img1" | default (.Get "url") | default (.Get 0)
+}}{{ $url2 := .Get "img2" | default (.Get "fallback") | default (.Get 1)
+}}{{ $alt := .Get "alt" | default (.Get 2)
+}}
+
+<!-- These do not seem to work. Using JS solution.
+<picture>
+  <source srcset="{{ $url1 }}">
+  <img src="{{ $url2 }}" alt="{{ $alt }}">
+</picture>
+<picture>
+  <source media="(min-width: 1px)" srcset="{{ $url1 }}">
+  <img src="{{ $url2 }}" alt="{{ $alt }}">
+</picture>
+-->
+
+<img src="{{ $url1 }}" alt="{{ $alt }}" onerror="this.onerror=null; this.src='{{ $url2 }}';">